diff --git a/docs/generators/protobuf-schema.md b/docs/generators/protobuf-schema.md
index 104a47fe59c..53e5334ef97 100644
--- a/docs/generators/protobuf-schema.md
+++ b/docs/generators/protobuf-schema.md
@@ -7,6 +7,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 
 | Option | Description | Values | Default |
 | ------ | ----------- | ------ | ------- |
+|startEnumsWithUnknown|Introduces "UNKNOWN" as the first element of enumerations.| |false|
 
 ## IMPORT MAPPING
 
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
index 58ec9c56b67..30a8b4c3c32 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java
@@ -47,10 +47,14 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
 
     private static final String IMPORTS = "imports";
 
+    public static final String START_ENUMS_WITH_UNKNOWN = "startEnumsWithUnknown";
+
     private final Logger LOGGER = LoggerFactory.getLogger(ProtobufSchemaCodegen.class);
 
     protected String packageName = "openapitools";
 
+    private boolean startEnumsWithUnknown = false;
+
     @Override
     public CodegenType getTag() {
         return CodegenType.SCHEMA;
@@ -145,6 +149,7 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
 
         cliOptions.clear();
 
+        addSwitch(START_ENUMS_WITH_UNKNOWN, "Introduces \"UNKNOWN\" as the first element of enumerations.", startEnumsWithUnknown);
     }
 
     @Override
@@ -164,6 +169,10 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
             setPackageName((String) additionalProperties.get(CodegenConstants.PACKAGE_NAME));
         }
 
+        if (additionalProperties.containsKey(this.START_ENUMS_WITH_UNKNOWN)) {
+            this.startEnumsWithUnknown = convertPropertyToBooleanAndWriteBack(START_ENUMS_WITH_UNKNOWN);
+        }
+
         supportingFiles.add(new SupportingFile("README.mustache", "", "README.md"));
     }
 
@@ -183,7 +192,27 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
         return camelize(sanitizeName(operationId));
     }
 
-    public void addEnumIndexes(List<Map<String, Object>> enumVars){
+    public void addUnknownToAllowableValues(Map<String, Object> allowableValues, String name) {
+        if(startEnumsWithUnknown) {
+            if(allowableValues.containsKey("enumVars")) {
+                List<Map<String, Object>> enumVars = (List<Map<String, Object>>)allowableValues.get("enumVars");
+                
+                HashMap<String, Object> unknown = new HashMap<String, Object>();
+                unknown.put("name", name + "_UNKNOWN");
+                unknown.put("isString", "false");
+                unknown.put("value", "\"" + name + "_UNKNOWN\"");
+
+                enumVars.add(0, unknown);
+            }
+
+            if(allowableValues.containsKey("values")) {
+                List<String> values = (List<String>)allowableValues.get("values");           
+                values.add(0, name + "_UNKNOWN");
+            }
+        }
+    }
+
+    public void addEnumIndexes(List<Map<String, Object>> enumVars) {
         int enumIndex = 0;
         for (Map<String, Object> enumVar : enumVars) {
             enumVar.put("protobuf-enum-index", enumIndex);
@@ -202,6 +231,8 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
 
             if(cm.isEnum) {
                 Map<String, Object> allowableValues = cm.getAllowableValues();
+                addUnknownToAllowableValues(allowableValues, cm.getClassname());
+
                 if (allowableValues.containsKey("enumVars")) {
                     List<Map<String, Object>> enumVars = (List<Map<String, Object>>)allowableValues.get("enumVars");
                     addEnumIndexes(enumVars);
@@ -224,9 +255,13 @@ public class ProtobufSchemaCodegen extends DefaultCodegen implements CodegenConf
                     }
                 }
 
-                if (var.isEnum && var.allowableValues.containsKey("enumVars")) {
-                    List<Map<String, Object>> enumVars = (List<Map<String, Object>>)var.allowableValues.get("enumVars");
-                    addEnumIndexes(enumVars);
+                if (var.isEnum) {
+                    addUnknownToAllowableValues(var.allowableValues, var.getEnumName());
+                
+                    if(var.allowableValues.containsKey("enumVars")) {
+                        List<Map<String, Object>> enumVars = (List<Map<String, Object>>) var.allowableValues.get("enumVars");
+                        addEnumIndexes(enumVars);
+                    }
                 }
 
                 // Add x-protobuf-index, unless already specified