You just received a .md file — maybe from a colleague, downloaded from GitHub, or generated by an AI tool — and double-clicking it opened it in TextEdit as a wall of symbols. ## where headings should be. ** around bold words. What's going on?

This guide explains what .md files are, why you're seeing more of them, and the best ways to open and read them properly on a Mac.

What Is a .md File?

A .md file is a markdown file — a plain text document written with a simple formatting syntax. Instead of using a word processor like Microsoft Word, markdown uses lightweight symbols to indicate formatting:

# Heading 1, ## Heading 2 for headings
**bold** and *italic* for text styling
- item for bullet lists
`code` for inline code
```code block``` for multi-line code
[link text](URL) for links

When you render a markdown file in the right tool, all those symbols disappear and you see a beautifully formatted document with proper headings, lists, bold text, and code blocks. When you open it in TextEdit, you see the raw symbols — because TextEdit doesn't know how to render markdown.

Why You're Seeing More .md Files

Five years ago, markdown files were mostly a developer thing. READMEs on GitHub, documentation for open-source projects. If you weren't a developer, you probably never encountered a .md file.

That's changed. Here's why .md files are now everywhere:

AI tools generate them constantly. Claude Code, ChatGPT, GitHub Copilot, Cursor, and Windsurf all produce markdown output. Ask Claude Code to document your project — it writes a CLAUDE.md file. Ask ChatGPT to outline a plan — it delivers markdown. Export your conversation from any AI tool — markdown. These tools have collectively put millions of .md files onto people's computers who've never thought about markdown before.

GitHub and developer culture. Every software project on GitHub has a README.md. If someone has sent you a link to a project or included docs in a folder, there's a good chance it's markdown.

Documentation tools. Notion, Confluence, Linear, and many other tools export to markdown. If you're getting files from developers or technical teams, .md is a common format.

How to Open .md Files on Mac

Option 1: TextEdit (Not Recommended)

You can open any .md file in TextEdit — it's built in and already set as the default for many text files. But TextEdit shows you the raw markdown syntax, not the rendered output. For quick reading or editing of a file you know well, it works. For actually reading a formatted document, it's frustrating.

Option 2: VS Code (Overkill for Reading)

If you have Visual Studio Code installed, you can open .md files and use the built-in preview (⌘K V, or click the preview icon in the top right). The preview renders correctly and handles most markdown syntax.

The downside: VS Code is a full code editor. It's a 400MB Electron app that launches slowly and brings along file trees, status bars, and a UI built for writing code — not reading documents. For opening a single .md file to read it, VS Code is like using Photoshop to view a JPEG.

Option 3: A Dedicated Markdown Editor (Best)

A dedicated markdown editor is built specifically to open .md files and render them beautifully. You open the file, you see the formatted output. No setup, no extensions.

OpenMark is a native macOS markdown editor built with SwiftUI for macOS Tahoe. Open a .md file and it renders instantly — headings, lists, code blocks, tables, Mermaid diagrams, and LaTeX math. Toggle to edit mode when you need to make changes. It's $9.99 one-time from the Mac App Store.

How to Set OpenMark as Your Default App for .md Files

Once you have OpenMark installed, you can make it the default so that double-clicking any .md file opens it automatically:

Find any .md file in Finder
Right-click (or Control-click) on it
Select Get Info (or press ⌘I)
In the Info window, find the Open With section
Click the dropdown and select OpenMark
Click Change All… to apply to all .md files
Confirm when asked

From now on, every .md file you double-click will open directly in OpenMark, rendered and readable.

What About Other File Extensions?

Markdown files sometimes use other extensions:

.markdown — same as .md, just more explicit
.mdown — another variant, less common
.mkd and .mkdn — occasionally used by older tools
.mdx — MDX files (markdown with embedded JSX, used in React documentation)

OpenMark handles .md, .markdown, .mdown, .mkd, and .mkdn. If you're seeing .mdx files, those are for developers building React documentation sites and typically live in code projects.

Summary

.md files are plain text formatted with markdown syntax. They need a markdown-aware viewer to look right. TextEdit shows the raw symbols. VS Code can preview them but it's heavy. A dedicated markdown editor like OpenMark opens and renders them instantly.

If you're running macOS Tahoe, OpenMark is the fastest way to go from double-clicking a .md file to reading it properly — rendered headings, formatted tables, working code blocks.

For more on why there are so many markdown files around now, see The Best Markdown Editor for AI-Generated Content. And if you're a developer who works with markdown regularly, Markdown for Developers: README Files, Documentation, and More covers the features most people miss.

Download OpenMark → — $9.99, one-time, native macOS. Open any .md file and see it rendered beautifully.