diff --git a/core/src/main/java/com/facebook/ktfmt/Main.kt b/core/src/main/java/com/facebook/ktfmt/Main.kt
index 62681246..f5845835 100644
--- a/core/src/main/java/com/facebook/ktfmt/Main.kt
+++ b/core/src/main/java/com/facebook/ktfmt/Main.kt
@@ -121,7 +121,10 @@ fun expandArgsToFileNames(args: List<String>): List<File> {
     if (arg == "-") {
       error("Error: '-', which causes ktfmt to read from stdin, should not be mixed with file name")
     }
-    result.addAll(File(arg).walkTopDown().filter { it.isFile && it.extension == "kt" })
+    result.addAll(
+        File(arg).walkTopDown().filter {
+          it.isFile && (it.extension == "kt" || it.extension == "kts")
+        })
   }
   return result
 }
diff --git a/core/src/test/java/com/facebook/ktfmt/MainKtTest.kt b/core/src/test/java/com/facebook/ktfmt/MainKtTest.kt
index 72f6d643..c9e3c72e 100644
--- a/core/src/test/java/com/facebook/ktfmt/MainKtTest.kt
+++ b/core/src/test/java/com/facebook/ktfmt/MainKtTest.kt
@@ -216,4 +216,25 @@ class MainKtTest {
 
     assertThat(output.toString("UTF-8")).isEqualTo(code)
   }
+
+  @Test
+  fun `expandArgsToFileNames - resolves 'kt' and 'kts' filenames only (recursively)`() {
+    val f1 = root.resolve("1.kt")
+    val f2 = root.resolve("2.kt")
+    val f3 = root.resolve("3")
+    val f4 = root.resolve("4.dummyext")
+    val f5 = root.resolve("5.kts")
+
+    val dir = root.resolve("foo")
+    dir.mkdirs()
+    val f6 = root.resolve("foo/1.kt")
+    val f7 = root.resolve("foo/2.kts")
+    val f8 = root.resolve("foo/3.dummyext")
+    val files = listOf(f1, f2, f3, f4, f5, f6, f7, f8)
+    for (f in files) {
+      f.createNewFile()
+    }
+    assertThat(expandArgsToFileNames(files.map { it.toString() }))
+        .containsExactly(f1, f2, f5, f6, f7)
+  }
 }