diff --git a/src/renderer/src/App.tsx b/src/renderer/src/App.tsx
index a79bcd0..c900711 100644
--- a/src/renderer/src/App.tsx
+++ b/src/renderer/src/App.tsx
@@ -18,6 +18,7 @@ import { DeviceDetailScreen } from "./components/DeviceDetailScreen";
 import { SettingsScreen } from "./components/SettingsScreen";
 import { ScenesScreen } from "./components/ScenesScreen";
 import { HideOnScroll } from "./components/HideOnScroll";
+import { EnvironmentStatusBar } from "./components/EnvironmentStatusBar";
 import { AppDispatch } from "./store/store";
 import { createAppTheme } from "./theme";
 import {
@@ -29,7 +30,14 @@ import {
   testApiCredentials,
   selectTheme,
 } from "./store/slices/settingsSlice";
-import { loadDeviceOrder, pollAllDeviceStatuses, selectAllDevices } from "./store/slices/deviceSlice"; // Added pollAllDeviceStatuses
+import {
+  clearDevices,
+  clearSelectedDeviceStatus,
+  fetchDevices,
+  loadDeviceOrder,
+  pollAllDeviceStatuses,
+  selectAllDevices,
+} from "./store/slices/deviceSlice";
 import { loadNightLightSceneAssignments, loadSceneOrder } from "./store/slices/sceneSlice";
 import { useTranslation } from "./useTranslation";
 
@@ -106,6 +114,17 @@ function App() {
     dispatch(testApiCredentials());
   }, [dispatch, storedToken, storedSecret]);
 
+  // Fetch device list at the App level so the environment status bar (and
+  // polling) work regardless of which view is active on startup.
+  useEffect(() => {
+    if (isTokenValid && storedToken) {
+      dispatch(fetchDevices());
+      return;
+    }
+    dispatch(clearDevices());
+    dispatch(clearSelectedDeviceStatus());
+  }, [dispatch, isTokenValid, storedToken]);
+
   // Effect for polling logic
   useEffect(() => {
     // Clear any existing timer
@@ -204,9 +223,12 @@ function App() {
         <HideOnScroll>
           <AppBar position="fixed" color="default" elevation={0}>
             <Toolbar>
-              <Typography variant="h6" component="div" sx={{ flexGrow: 1, fontWeight: "bold", letterSpacing: -0.5 }}>
+              <Typography variant="h6" component="div" sx={{ fontWeight: "bold", letterSpacing: -0.5 }}>
                 {t("App Title")}
               </Typography>
+              <Box sx={{ flexGrow: 1, display: "flex", justifyContent: "center" }}>
+                <EnvironmentStatusBar />
+              </Box>
               <Box sx={{ display: 'flex', gap: 1 }}>
                 <Button
                   color={currentView === "list" ? "primary" : "inherit"}
diff --git a/src/renderer/src/components/DeviceListScreen.tsx b/src/renderer/src/components/DeviceListScreen.tsx
index d04bdfe..ece57a0 100644
--- a/src/renderer/src/components/DeviceListScreen.tsx
+++ b/src/renderer/src/components/DeviceListScreen.tsx
@@ -16,7 +16,6 @@ import {
   selectAllDevices,
   selectDevicesLoading,
   selectDevicesError,
-  clearDevices,
   setError,
   selectDeviceStatusMap,
   setDeviceOrder,
@@ -120,14 +119,12 @@ export const DeviceListScreen: React.FC<{ onDeviceSelect: (deviceId: string) =>
     []
   );
 
+  // Device fetching is handled at the App level. This effect only sets
+  // contextual error messages for the list view.
   useEffect(() => {
-    if (isTokenValid && apiTokenSet) {
-      dispatch(fetchDevices());
-    } else if (apiTokenSet && !isTokenValid) {
-      dispatch(clearDevices());
+    if (apiTokenSet && !isTokenValid) {
       dispatch(setError(t("API credentials are set but not validated. Please test them in Settings.")));
-    } else {
-      dispatch(clearDevices());
+    } else if (!apiTokenSet) {
       dispatch(setError(t("API credentials not configured. Please go to Settings.")));
     }
   }, [dispatch, isTokenValid, apiTokenSet, t]);
diff --git a/src/renderer/src/components/EnvironmentStatusBar.tsx b/src/renderer/src/components/EnvironmentStatusBar.tsx
new file mode 100644
index 0000000..2386030
--- /dev/null
+++ b/src/renderer/src/components/EnvironmentStatusBar.tsx
@@ -0,0 +1,176 @@
+import React, { useState } from "react";
+import { useDispatch, useSelector } from "react-redux";
+import Box from "@mui/material/Box";
+import ButtonBase from "@mui/material/ButtonBase";
+import Typography from "@mui/material/Typography";
+import Menu from "@mui/material/Menu";
+import MenuItem from "@mui/material/MenuItem";
+import ListItemText from "@mui/material/ListItemText";
+import ListItemIcon from "@mui/material/ListItemIcon";
+import Divider from "@mui/material/Divider";
+import ThermostatIcon from "@mui/icons-material/Thermostat";
+import WaterDropIcon from "@mui/icons-material/WaterDrop";
+import Co2Icon from "@mui/icons-material/Co2";
+import ArrowDropDownIcon from "@mui/icons-material/ArrowDropDown";
+import CheckIcon from "@mui/icons-material/Check";
+import CloseIcon from "@mui/icons-material/Close";
+
+import { AppDispatch } from "../store/store";
+import { selectAllDevices, selectDeviceStatusMap } from "../store/slices/deviceSlice";
+import {
+  selectPinnedEnvironmentDeviceId,
+  setPinnedEnvironmentDeviceId,
+} from "../store/slices/settingsSlice";
+import { findDeviceDefinition, getStatusFieldsForDevice } from "../deviceDefinitions";
+import { useTranslation } from "../useTranslation";
+
+/** Device definition keys that provide temperature/humidity readings. */
+const ENVIRONMENT_DEVICE_KEYS = new Set(["meter", "hub2", "hub3"]);
+
+export const EnvironmentStatusBar: React.FC = () => {
+  const dispatch: AppDispatch = useDispatch();
+  const { t } = useTranslation();
+  const devices = useSelector(selectAllDevices);
+  const statusMap = useSelector(selectDeviceStatusMap);
+  const pinnedId = useSelector(selectPinnedEnvironmentDeviceId);
+  const [anchorEl, setAnchorEl] = useState<null | HTMLElement>(null);
+
+  const envDevices = devices.filter((device) => {
+    const def = findDeviceDefinition(device.deviceType);
+    return def && ENVIRONMENT_DEVICE_KEYS.has(def.key);
+  });
+
+  // Nothing to show if no environment-capable devices exist
+  if (envDevices.length === 0) return null;
+
+  const pinnedDevice = pinnedId ? envDevices.find((d) => d.deviceId === pinnedId) : undefined;
+
+  const getFormattedValue = (deviceId: string, fieldKey: string): string | undefined => {
+    const device = devices.find((d) => d.deviceId === deviceId);
+    if (!device) return undefined;
+    const statusFields = getStatusFieldsForDevice(device.deviceType);
+    const field = statusFields.find((f) => f.key === fieldKey);
+    if (!field) return undefined;
+    const status = statusMap[deviceId];
+    const raw = status ? (status as any)[field.key] : undefined;
+    if (raw === undefined || raw === null) return undefined;
+    return field.formatter ? String(field.formatter(raw, status)) : String(raw);
+  };
+
+  const temp = pinnedDevice ? getFormattedValue(pinnedDevice.deviceId, "temperature") : undefined;
+  const hum = pinnedDevice ? getFormattedValue(pinnedDevice.deviceId, "humidity") : undefined;
+  const co2 = pinnedDevice ? getFormattedValue(pinnedDevice.deviceId, "CO2") : undefined;
+
+  const handleOpen = (e: React.MouseEvent<HTMLElement>) => setAnchorEl(e.currentTarget);
+  const handleClose = () => setAnchorEl(null);
+
+  const handleSelect = (deviceId: string) => {
+    dispatch(setPinnedEnvironmentDeviceId(deviceId));
+    handleClose();
+  };
+
+  const handleClear = () => {
+    dispatch(setPinnedEnvironmentDeviceId(null));
+    handleClose();
+  };
+
+  return (
+    <>
+      <ButtonBase
+        onClick={handleOpen}
+        aria-label={t("Select sensor")}
+        aria-haspopup="listbox"
+        sx={{
+          display: "flex",
+          alignItems: "center",
+          gap: 0.5,
+          px: 1.5,
+          py: 0.5,
+          borderRadius: 2,
+          "&:hover": { bgcolor: "action.hover" },
+        }}
+      >
+        {pinnedDevice && (temp || hum || co2) ? (
+          <>
+            {temp && (
+              <Box sx={{ display: "flex", alignItems: "center", gap: 0.3 }}>
+                <ThermostatIcon sx={{ fontSize: 18, color: "warning.main" }} />
+                <Typography variant="body2" fontWeight="bold">
+                  {temp}
+                </Typography>
+              </Box>
+            )}
+            {hum && (
+              <Box sx={{ display: "flex", alignItems: "center", gap: 0.3, ml: 1 }}>
+                <WaterDropIcon sx={{ fontSize: 18, color: "info.main" }} />
+                <Typography variant="body2" fontWeight="bold">
+                  {hum}
+                </Typography>
+              </Box>
+            )}
+            {co2 && (
+              <Box sx={{ display: "flex", alignItems: "center", gap: 0.3, ml: 1 }}>
+                <Co2Icon sx={{ fontSize: 20, color: "success.main" }} />
+                <Typography variant="body2" fontWeight="bold">
+                  {co2}
+                </Typography>
+              </Box>
+            )}
+          </>
+        ) : (
+          <Typography variant="body2" color="text.secondary">
+            {pinnedDevice ? "—" : t("Select sensor")}
+          </Typography>
+        )}
+        <ArrowDropDownIcon sx={{ fontSize: 18, color: "text.secondary" }} />
+      </ButtonBase>
+
+      <Menu
+        anchorEl={anchorEl}
+        open={Boolean(anchorEl)}
+        onClose={handleClose}
+        anchorOrigin={{ vertical: "bottom", horizontal: "left" }}
+        transformOrigin={{ vertical: "top", horizontal: "left" }}
+        slotProps={{ paper: { sx: { minWidth: 220 } } }}
+      >
+        {envDevices.map((device) => {
+          const isSelected = device.deviceId === pinnedId;
+          const dTemp = getFormattedValue(device.deviceId, "temperature");
+          const dHum = getFormattedValue(device.deviceId, "humidity");
+          const dCo2 = getFormattedValue(device.deviceId, "CO2");
+          const preview = [dTemp ?? "—", dHum ?? "—", ...(dCo2 ? [dCo2] : [])].join(" / ");
+          return (
+            <MenuItem
+              key={device.deviceId}
+              selected={isSelected}
+              onClick={() => handleSelect(device.deviceId)}
+            >
+              {isSelected && (
+                <ListItemIcon>
+                  <CheckIcon fontSize="small" />
+                </ListItemIcon>
+              )}
+              <ListItemText inset={!isSelected}>
+                {device.deviceName || t("Unnamed Device")}
+              </ListItemText>
+              <Typography variant="caption" color="text.secondary" sx={{ ml: 2 }}>
+                {preview}
+              </Typography>
+            </MenuItem>
+          );
+        })}
+        {pinnedId && (
+          <>
+            <Divider />
+            <MenuItem onClick={handleClear}>
+              <ListItemIcon>
+                <CloseIcon fontSize="small" />
+              </ListItemIcon>
+              <ListItemText>{t("Clear selection")}</ListItemText>
+            </MenuItem>
+          </>
+        )}
+      </Menu>
+    </>
+  );
+};
diff --git a/src/renderer/src/i18n.ts b/src/renderer/src/i18n.ts
index b575420..2f9a3ec 100644
--- a/src/renderer/src/i18n.ts
+++ b/src/renderer/src/i18n.ts
@@ -41,6 +41,8 @@ const en: Dictionary = {
   "Cannot refresh. API credentials are not valid or not set.":
     "Cannot refresh. API credentials are not valid or not set.",
   "No devices found, or check API permissions.": "No devices found, or check API permissions.",
+  "Select sensor": "Select sensor",
+  "Clear selection": "Clear selection",
   "Unnamed Device": "Unnamed Device",
   Unknown: "Unknown",
   Details: "Details",
@@ -206,6 +208,8 @@ const ja: Dictionary = {
     "再読み込みできません。API 認証情報が未設定または無効です。",
   "No devices found, or check API permissions.":
     "デバイスが見つかりません。API 権限を確認してください。",
+  "Select sensor": "センサーを選択",
+  "Clear selection": "選択を解除",
   "Unnamed Device": "名称未設定デバイス",
   Unknown: "不明",
   Details: "詳細",
diff --git a/src/renderer/src/store/slices/__tests__/settingsSlice.test.ts b/src/renderer/src/store/slices/__tests__/settingsSlice.test.ts
index 33ff44c..98eb89b 100644
--- a/src/renderer/src/store/slices/__tests__/settingsSlice.test.ts
+++ b/src/renderer/src/store/slices/__tests__/settingsSlice.test.ts
@@ -16,6 +16,7 @@ describe('settingsSlice reducers', () => {
     theme: 'system',
     logRetentionDays: 7,
     language: 'en',
+    pinnedEnvironmentDeviceId: null,
   };
 
   it('should handle initial state', () => {
diff --git a/src/renderer/src/store/slices/settingsSlice.ts b/src/renderer/src/store/slices/settingsSlice.ts
index dc9851c..b5d8b5c 100644
--- a/src/renderer/src/store/slices/settingsSlice.ts
+++ b/src/renderer/src/store/slices/settingsSlice.ts
@@ -13,6 +13,7 @@ export interface SettingsState {
   theme: "light" | "dark" | "system";
   logRetentionDays: number;
   language: "en" | "ja";
+  pinnedEnvironmentDeviceId: string | null;
 }
 
 const mockDefaults = {
@@ -30,6 +31,7 @@ const initialState: SettingsState = {
   theme: "system",
   logRetentionDays: 7,
   language: "en",
+  pinnedEnvironmentDeviceId: null,
 };
 
 const persistSetting = (key: string, value: unknown, label: string) => {
@@ -91,6 +93,10 @@ export const loadApiCredentials = createAsyncThunk(
       if (storedLanguage === "en" || storedLanguage === "ja") {
         dispatch(setLanguage(storedLanguage));
       }
+      const storedPinnedDevice = await window.electronStore.get("pinnedEnvironmentDeviceId");
+      if (typeof storedPinnedDevice === "string") {
+        dispatch(setPinnedEnvironmentDeviceId(storedPinnedDevice));
+      }
 
       if (token && secret) {
         dispatch(setApiCredentials({ token, secret }));
@@ -276,7 +282,14 @@ export const settingsSlice = createSlice({
       state.language = action.payload;
       persistSetting("language", action.payload, "language");
     },
-    // ... other reducers
+    setPinnedEnvironmentDeviceId: (state, action: PayloadAction<string | null>) => {
+      state.pinnedEnvironmentDeviceId = action.payload;
+      if (action.payload) {
+        persistSetting("pinnedEnvironmentDeviceId", action.payload, "pinned environment device");
+      } else {
+        deleteSetting("pinnedEnvironmentDeviceId", "pinned environment device");
+      }
+    },
   },
   extraReducers: (builder) => {
     builder
@@ -307,6 +320,7 @@ export const {
   setPollingInterval,
   setTheme,
   setLanguage,
+  setPinnedEnvironmentDeviceId,
 } = settingsSlice.actions;
 
 export const selectApiToken = (state: RootState) => state.settings.apiToken;
@@ -316,5 +330,6 @@ export const selectValidationMessage = (state: RootState) => state.settings.vali
 export const selectPollingInterval = (state: RootState) => state.settings.pollingIntervalSeconds;
 export const selectTheme = (state: RootState) => state.settings.theme;
 export const selectLanguage = (state: RootState) => state.settings.language;
+export const selectPinnedEnvironmentDeviceId = (state: RootState) => state.settings.pinnedEnvironmentDeviceId;
 
 export default settingsSlice.reducer;