diff --git a/json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java b/json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java
index 3a31151f..a9f7df5f 100644
--- a/json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java
+++ b/json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java
@@ -14,7 +14,7 @@ import com.jayway.jsonpath.internal.function.sequence.Last;
 import com.jayway.jsonpath.internal.function.text.Concatenate;
 import com.jayway.jsonpath.internal.function.text.Length;
 
-import java.util.Collections;
+import java.io.InvalidClassException;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -28,7 +28,7 @@ import java.util.Map;
  */
 public class PathFunctionFactory {
 
-    public static final Map<String, Class> FUNCTIONS;
+    private static final Map<String, Class> FUNCTIONS;
 
     static {
         // New functions should be added here and ensure the name is not overridden
@@ -56,7 +56,7 @@ public class PathFunctionFactory {
         map.put("index", Index.class);
 
 
-        FUNCTIONS = Collections.unmodifiableMap(map);
+        FUNCTIONS = map;
     }
 
     /**
@@ -85,4 +85,14 @@ public class PathFunctionFactory {
             }
         }
     }
+
+    public static void addCustomFunction(String name,Class function) throws InvalidClassException {
+        if(FUNCTIONS.containsKey(name)){
+            throw new InvalidPathException("Function with name: " + name + " already exists");
+        }
+        if (!PathFunction.class.isAssignableFrom(function)){
+            throw new InvalidClassException("Function with name: " + name + "must be a instance of PathFunction class");
+        }
+        FUNCTIONS.put(name,function);
+    }
 }
diff --git a/json-path/src/test/java/com/jayway/jsonpath/internal/function/CustomFunctionTest.java b/json-path/src/test/java/com/jayway/jsonpath/internal/function/CustomFunctionTest.java
new file mode 100644
index 00000000..3c9593ca
--- /dev/null
+++ b/json-path/src/test/java/com/jayway/jsonpath/internal/function/CustomFunctionTest.java
@@ -0,0 +1,37 @@
+package com.jayway.jsonpath.internal.function;
+
+import com.jayway.jsonpath.Configuration;
+import com.jayway.jsonpath.Configurations;
+import com.jayway.jsonpath.InvalidPathException;
+import org.junit.jupiter.api.Test;
+
+import java.io.InvalidClassException;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+class CustomFunctionTest extends BaseFunctionTest {
+
+    private Configuration conf = Configurations.JSON_SMART_CONFIGURATION;
+
+    public class InvalidFunction {
+
+    }
+    @Test
+    void testAddInvalidFunction(){
+        assertThrows(InvalidClassException.class, () -> PathFunctionFactory.addCustomFunction("invalid",
+                InvalidFunction.class));
+    }
+
+    @Test
+    void testAddValidFunction(){
+       assertDoesNotThrow(()->PathFunctionFactory.addCustomFunction("toUpperCase",ToUpperCase.class));
+       verifyTextFunction(conf,"$['text'][0].toUpperCase()","A");
+    }
+
+    @Test
+    void testAddExistentFunction(){
+        assertThrows(InvalidPathException.class, () -> PathFunctionFactory.addCustomFunction("avg",
+                ToUpperCase.class));
+    }
+}
diff --git a/json-path/src/test/java/com/jayway/jsonpath/internal/function/ToUpperCase.java b/json-path/src/test/java/com/jayway/jsonpath/internal/function/ToUpperCase.java
new file mode 100644
index 00000000..6a7c4c1c
--- /dev/null
+++ b/json-path/src/test/java/com/jayway/jsonpath/internal/function/ToUpperCase.java
@@ -0,0 +1,14 @@
+package com.jayway.jsonpath.internal.function;
+
+import com.jayway.jsonpath.internal.EvaluationContext;
+import com.jayway.jsonpath.internal.PathRef;
+
+import java.util.List;
+
+public class ToUpperCase implements PathFunction{
+    @Override
+    public Object invoke(String currentPath, PathRef parent, Object model, EvaluationContext ctx,
+                         List<Parameter> parameters) {
+        return ((String) model).toUpperCase();
+    }
+}