diff --git a/system/jlib/jstring.cpp b/system/jlib/jstring.cpp
index 7e581f2503a..06d80b772e0 100644
--- a/system/jlib/jstring.cpp
+++ b/system/jlib/jstring.cpp
@@ -937,28 +937,35 @@ StringBuffer & StringBuffer::replace(char oldChar, char newChar)
 // Copy source to result, replacing all occurrences of "oldStr" with "newStr"
 StringBuffer &replaceString(StringBuffer & result, size_t lenSource, const char *source, size_t lenOldStr, const char* oldStr, size_t lenNewStr, const char* newStr)
 {
-    if (lenSource)
+    if (lenOldStr && lenSource >= lenOldStr)
     {
-        size_t left = lenSource;
-        while (left >= lenOldStr)
+        // Avoid allocating an unnecessarly large buffer and match the source string
+        result.ensureCapacity(lenSource);
+
+        size_t offset = 0;
+        size_t lastCopied = 0;
+        size_t maxOffset = lenSource - lenOldStr + 1;
+        char firstChar = oldStr[0];
+        while (offset < maxOffset)
         {
-            if (memcmp(source, oldStr, lenOldStr)==0)
+            if (unlikely(source[offset] == firstChar)
+                && unlikely((lenOldStr == 1) || memcmp(source + offset, oldStr, lenOldStr)==0))
             {
+                // If lastCopied matches the offset nothing is appended, but we can avoid a test for offset == lastCopied
+                result.append(offset - lastCopied, source + lastCopied);
                 result.append(lenNewStr, newStr);
-                source += lenOldStr;
-                left -= lenOldStr;
+                offset += lenOldStr;
+                lastCopied = offset;
             }
             else
-            {
-                result.append(*source);
-                source++;
-                left--;
-            }
+                offset++;
         }
-
-        // there are no more possible replacements, make sure we keep the end of the original buffer
-        result.append(left, source);
+        // Append the remaining characters
+        result.append(lenSource - lastCopied, source + lastCopied);
     }
+    else
+        result.append(lenSource, source); // Search string does not fit in source or is empty
+
     return result;
 }
 
diff --git a/testing/unittests/jlibtests.cpp b/testing/unittests/jlibtests.cpp
index a19ebe36159..5740207ac89 100644
--- a/testing/unittests/jlibtests.cpp
+++ b/testing/unittests/jlibtests.cpp
@@ -845,6 +845,7 @@ class JlibStringTest : public CppUnit::TestFixture
 public:
     CPPUNIT_TEST_SUITE(JlibStringTest);
         CPPUNIT_TEST(testEncodeCSVColumn);
+        CPPUNIT_TEST(testReplaceString);
     CPPUNIT_TEST_SUITE_END();
 
 protected:
@@ -860,6 +861,59 @@ void testEncodeCSVColumn()
         encodeCSVColumn(encodedCSV, csvCol2);
         CPPUNIT_ASSERT_EQUAL_STR(encodedCSV.str(), "\"hello world, \"\"how are you?\"\"\"");
     }
+
+    void testReplaceString()
+    {
+        // Match string at the end of the source
+        StringBuffer source("aaaaaaaaab");
+        source.replaceString("aaab", "x");
+        CPPUNIT_ASSERT_EQUAL_STR("aaaaaax", source.str());
+
+        // Single char match at end of source
+        source.set("aaaaaaaaab");
+        source.replaceString("b", "x");
+        CPPUNIT_ASSERT_EQUAL_STR("aaaaaaaaax", source.str());
+
+        // Match string at the start of the source
+        source.set("abababab");
+        source.replaceString("aba", "xxx");
+        CPPUNIT_ASSERT_EQUAL_STR("xxxbxxxb", source.str());
+
+        // Match not found
+        source.set("aaaaa");
+        source.replaceString("bb", "xxx");
+        CPPUNIT_ASSERT_EQUAL_STR("aaaaa", source.str());
+
+        // Replace string is empty
+        source.set("aabaa");
+        source.replaceString("aa", "");
+        CPPUNIT_ASSERT_EQUAL_STR("b", source.str());
+
+        // Search string is empty
+        source.set("aaaaaa");
+        source.replaceString("", "b");
+        CPPUNIT_ASSERT_EQUAL_STR("aaaaaa", source.str());
+
+        // Source string is empty
+        source.set("");
+        source.replaceString("a", "b");
+        CPPUNIT_ASSERT_EQUAL_STR("", source.str());
+
+        // Search string is longer than source string
+        source.set("a");
+        source.replaceString("aa", "b");
+        CPPUNIT_ASSERT_EQUAL_STR("a", source.str());
+
+        // Replace every character
+        source.set("aaaaa");
+        source.replaceString("a", "b");
+        CPPUNIT_ASSERT_EQUAL_STR("bbbbb", source.str());
+
+        // Replace string is longer than search string
+        source.set("abbabab");
+        source.replaceString("ab", "xxx");
+        CPPUNIT_ASSERT_EQUAL_STR("xxxbxxxxxx", source.str());
+    }
 };
 
 CPPUNIT_TEST_SUITE_REGISTRATION( JlibStringTest );