diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/api.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/api.mustache
index a1c2bf2771e..afd9ff0a5b4 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/api.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/api.mustache
@@ -66,7 +66,14 @@ import kotlinx.serialization.encoding.*
             {{#isMultipart}}
             formData {
                 {{#formParams}}
+                {{#isArray}}
+                {{{paramName}}}?.onEach {
+                    append("{{{baseName}}}[]", it)
+                }
+                {{/isArray}}
+                {{^isArray}}
                 {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}) }
+                {{/isArray}}
                 {{/formParams}}
             }
             {{/isMultipart}}
diff --git a/samples/client/petstore/kotlin-default-values-multiplatform/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt b/samples/client/petstore/kotlin-default-values-multiplatform/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt
index ab00c6d447c..1b7c26ff63f 100644
--- a/samples/client/petstore/kotlin-default-values-multiplatform/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt
+++ b/samples/client/petstore/kotlin-default-values-multiplatform/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt
@@ -81,7 +81,9 @@ open class DefaultApi(
                 fn1?.apply { append("fn1", fn1) }
                 fn2?.apply { append("fn2", fn2) }
                 fn3?.apply { append("fn3", fn3) }
-                fn4?.apply { append("fn4", fn4) }
+                fn4?.onEach {
+                    append("fn4[]", it)
+                }
             }
 
         val localVariableQuery = mutableMapOf<String, List<String>>()