Option 1 — A native reader (best for regular use)
Read.md for Mac is a reader-first markdown app: double-click a file and it opens formatted — headings, tables, syntax-highlighted code, Mermaid diagrams, light/dark themes, and a table of contents for long docs. It keeps recents, works fully offline, and can browse GitHub repositories directly.
Option 2 — The browser (no install)
For a one-off file, use the free web viewer: drag the file into Safari or Chrome and it renders immediately. It runs entirely in your browser — the file is never uploaded, so it's fine for work documents.
Option 3 — TextEdit (what you get by default)
macOS assigns .md files to TextEdit, which treats them as plain text: you'll see # headings and **asterisks** rather than formatting. Quick Look (space bar) does the same. It works in a pinch — you just won't want to read anything longer than a paragraph that way.
Tip: make .md files open in a reader by default
Right-click any .md file → Get Info → Open with → choose Read.md → Change All…. From then on, double-click opens the rendered document.