diff --git a/glow_test.go b/glow_test.go
index 8743be2..d45fa18 100644
--- a/glow_test.go
+++ b/glow_test.go
@@ -1,9 +1,48 @@
 package main
 
 import (
+	"bytes"
+	"io"
+	"strings"
 	"testing"
+
+	"github.com/spf13/cobra"
 )
 
+func TestExecuteCLIRendersMMDAsMarkdown(t *testing.T) {
+	const content = "# Title\n\nHello **world**\n"
+
+	oldStyle, oldWidth := style, width
+	oldPager, oldTUI := pager, tui
+	t.Cleanup(func() {
+		style, width = oldStyle, oldWidth
+		pager, tui = oldPager, oldTUI
+	})
+
+	style = "notty"
+	width = 80
+	pager = false
+	tui = false
+
+	render := func(name string) string {
+		t.Helper()
+
+		var out bytes.Buffer
+		src := &source{
+			reader: io.NopCloser(strings.NewReader(content)),
+			URL:    name,
+		}
+		if err := executeCLI(&cobra.Command{}, src, &out); err != nil {
+			t.Fatalf("executeCLI(%q): %v", name, err)
+		}
+		return out.String()
+	}
+
+	if got, want := render("test.mmd"), render("test.md"); got != want {
+		t.Fatalf(".mmd rendered differently than .md\n.mmd:\n%q\n.md:\n%q", got, want)
+	}
+}
+
 func TestGlowFlags(t *testing.T) {
 	tt := []struct {
 		args  []string
diff --git a/ui/ui.go b/ui/ui.go
index 3537d3f..826dceb 100644
--- a/ui/ui.go
+++ b/ui/ui.go
@@ -25,7 +25,7 @@ var (
 	config Config
 
 	markdownExtensions = []string{
-		"*.md", "*.mdown", "*.mkdn", "*.mkd", "*.markdown",
+		"*.md", "*.mdown", "*.mkdn", "*.mkd", "*.markdown", "*.mmd",
 	}
 )
 
diff --git a/utils/utils.go b/utils/utils.go
index 9f33a1e..f77a63b 100644
--- a/utils/utils.go
+++ b/utils/utils.go
@@ -46,7 +46,7 @@ func WrapCodeBlock(s, language string) string {
 }
 
 var markdownExtensions = []string{
-	".md", ".mdown", ".mkdn", ".mkd", ".markdown",
+	".md", ".mdown", ".mkdn", ".mkd", ".markdown", ".mmd",
 }
 
 // IsMarkdownFile returns whether the filename has a markdown extension.