diff --git a/cpp/src/gandiva/precompiled/string_ops.cc b/cpp/src/gandiva/precompiled/string_ops.cc
index 5aa0eb38eafd7..3849cf7bdf9a5 100644
--- a/cpp/src/gandiva/precompiled/string_ops.cc
+++ b/cpp/src/gandiva/precompiled/string_ops.cc
@@ -1377,7 +1377,7 @@ gdv_int32 ascii_utf8(const char* data, gdv_int32 data_len) {
   if (data_len == 0) {
     return 0;
   }
-  return static_cast<gdv_int32>(data[0]);
+  return static_cast<gdv_int32>(static_cast<signed char>(data[0]));
 }
 
 // Returns the ASCII character having the binary equivalent to A.
diff --git a/cpp/src/gandiva/precompiled/string_ops_test.cc b/cpp/src/gandiva/precompiled/string_ops_test.cc
index 89213592e7ea2..aaa25db0a9f8d 100644
--- a/cpp/src/gandiva/precompiled/string_ops_test.cc
+++ b/cpp/src/gandiva/precompiled/string_ops_test.cc
@@ -51,6 +51,8 @@ TEST(TestStringOps, TestAscii) {
   EXPECT_EQ(ascii_utf8("", 0), 0);
   EXPECT_EQ(ascii_utf8("123", 3), 49);
   EXPECT_EQ(ascii_utf8("999", 3), 57);
+  EXPECT_EQ(ascii_utf8("\x80", 1), -128);
+  EXPECT_EQ(ascii_utf8("\xFF", 1), -1);
 }
 
 TEST(TestStringOps, TestChrBigInt) {