diff --git a/cpp/src/gandiva/precompiled/string_ops.cc b/cpp/src/gandiva/precompiled/string_ops.cc
index b7c6b518b45..d31e8002758 100644
--- a/cpp/src/gandiva/precompiled/string_ops.cc
+++ b/cpp/src/gandiva/precompiled/string_ops.cc
@@ -3099,6 +3099,19 @@ static char mappings[] = {'0', '1', '2', '3', '0', '1', '2', '0', '0',
 //    4. If the string have too few letters in the word that you can't assign three
 //    numbers, append with zeros until there are three numbers. If you have four or more
 //    numbers, retain only the first three.
+//
+// The mappings table below is defined only for the 26 ASCII letters, so the
+// classification and case-folding used to index it must stay ASCII-only. The
+// locale-sensitive isalpha/toupper let bytes 0x80-0xFF count as letters under a
+// non-C locale, and toupper on such a byte yields an index past mappings.
+static FORCE_INLINE bool is_ascii_letter(char c) {
+  return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
+}
+
+static FORCE_INLINE char ascii_toupper(char c) {
+  return (c >= 'a' && c <= 'z') ? static_cast<char>(c - ('a' - 'A')) : c;
+}
+
 FORCE_INLINE
 const char* soundex_utf8(gdv_int64 context, const char* in, gdv_int32 in_len,
                          bool in_validity, bool* out_valid, int32_t* out_len) {
@@ -3125,9 +3138,9 @@ const char* soundex_utf8(gdv_int64 context, const char* in, gdv_int32 in_len,
 
   int start_idx = 0;
   for (int i = 0; i < in_len; ++i) {
-    if (isalpha(static_cast<unsigned char>(in[i])) > 0) {
+    if (is_ascii_letter(in[i])) {
       // Retain the first letter
-      ret[0] = toupper(static_cast<unsigned char>(in[i]));
+      ret[0] = ascii_toupper(in[i]);
       start_idx = i + 1;
       break;
     }
@@ -3143,8 +3156,8 @@ const char* soundex_utf8(gdv_int64 context, const char* in, gdv_int32 in_len,
   soundex[0] = '\0';
   // Replace consonants with digits and special letters with 0
   for (int i = start_idx; i < in_len; i++) {
-    if (isalpha(static_cast<unsigned char>(in[i])) > 0) {
-      c = toupper(static_cast<unsigned char>(in[i])) - 65;
+    if (is_ascii_letter(in[i])) {
+      c = ascii_toupper(in[i]) - 'A';
       if (mappings[c] != soundex[si - 1]) {
         soundex[si] = mappings[c];
         si++;
diff --git a/cpp/src/gandiva/precompiled/string_ops_test.cc b/cpp/src/gandiva/precompiled/string_ops_test.cc
index 3faeb316231..695b0200447 100644
--- a/cpp/src/gandiva/precompiled/string_ops_test.cc
+++ b/cpp/src/gandiva/precompiled/string_ops_test.cc
@@ -18,9 +18,12 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
+#include <cctype>
+#include <clocale>
 #include <cstring>
 #include <limits>
 #include <memory>
+#include <string>
 
 #include "gandiva/execution_context.h"
 #include "gandiva/precompiled/types.h"
@@ -3046,6 +3049,51 @@ TEST(TestStringOps, TestSoundex) {
   EXPECT_EQ(validity, true);
 }
 
+TEST(TestStringOps, TestSoundexNonAsciiNoOverread) {
+  gandiva::ExecutionContext ctx;
+  auto ctx_ptr = reinterpret_cast<int64_t>(&ctx);
+  int32_t out_len = 0;
+  bool validity = false;
+  const char* out;
+
+  // ASCII input is unaffected by the fix.
+  out = soundex_utf8(ctx_ptr, "Robert", 6, true, &validity, &out_len);
+  EXPECT_EQ(std::string(out, out_len), "R163");
+
+  // The mappings table has 26 entries (A-Z). Under a non-C locale isalpha
+  // accepts bytes 0x80-0xFF and toupper leaves them past 'Z', which the old
+  // code used to index the table out of bounds. Find such a byte if the
+  // platform provides a locale for it, then confirm it is now treated as a
+  // separator instead of over-reading.
+  const char* candidates[] = {"en_US.UTF-8", "en_US.ISO8859-1", "C.UTF-8",
+                              "de_DE.ISO8859-1", ""};
+  std::string saved = setlocale(LC_CTYPE, nullptr);
+  int trigger = -1;
+  for (const char* loc : candidates) {
+    if (setlocale(LC_CTYPE, loc) == nullptr) {
+      continue;
+    }
+    for (int b = 0x80; b < 0x100; ++b) {
+      if (isalpha(b) && (toupper(b) - 'A' < 0 || toupper(b) - 'A' > 25)) {
+        trigger = b;
+        break;
+      }
+    }
+    if (trigger >= 0) {
+      break;
+    }
+  }
+
+  if (trigger >= 0) {
+    char buf[] = {'R', 'o', 'b', static_cast<char>(trigger), 'e', 'r', 't'};
+    out = soundex_utf8(ctx_ptr, buf, static_cast<int32_t>(sizeof(buf)), true, &validity,
+                       &out_len);
+    EXPECT_EQ(std::string(out, out_len), "R163");
+    EXPECT_EQ(validity, true);
+  }
+  setlocale(LC_CTYPE, saved.c_str());
+}
+
 TEST(TestStringOps, TestInstr) {
   std::string s1 = "hello world!";
   auto s1_len = static_cast<int32_t>(s1.size());