Mac Keyboard Typing Wrong Characters? Here's How to Fix It

You press a key and something else appears: " instead of @, § where ` should be, letters that don't match the keycaps, or full lines of gibberish. On a Mac this is almost never a broken keyboard - it's one of a handful of software causes. Go through them in order; the first three cover most cases.

1. The wrong input source is active

The most common cause. macOS switches layouts with Ctrl+Space or the Fn/Globe key, and it's easy to hit either without noticing - especially since macOS can also switch the layout by itself when you change apps.

Check the layout indicator in the menu bar. If it shows the wrong language, switch back.
To stop macOS from switching on its own, open System Settings → Keyboard → Text Input → Edit… and turn off "Automatically switch to a document's input source".

If you've already typed a sentence before noticing, you don't have to retype it: Boomkey converts the wrongly typed text in place when you double-tap Shift.

2. A different variant of the same layout

Layouts come in variants that look identical in the menu bar but put symbols in different places:

U.S. vs. British - on British, Shift+2 gives " instead of @, and Shift+3 gives £ instead of #.
ABC vs. U.S. - mostly the same, but easy to mix up with an international variant.
Ukrainian vs. Ukrainian - PC, Russian vs. Russian - PC - punctuation and some letters sit on different keys.

Open System Settings → Keyboard → Text Input → Edit… and check exactly which variant is in the list. Remove the wrong one with − and add the one matching your keycaps with +.

3. The keyboard type is detected wrong

macOS asks you to press a few keys when it first sees an external keyboard, and it can guess the standard wrong - an ISO (European) keyboard detected as ANSI (US) swaps ` and §, and ~ ends up in the wrong place.

Go to System Settings → Keyboard, connect the keyboard, and look for Change Keyboard Type… - the assistant reruns the detection. (On recent macOS versions this appears when an external keyboard is connected.)

4. A stuck or remapped modifier key

If every key produces symbols or shortcuts instead of letters, a modifier is involved:

Sticky Keys may be on: check System Settings → Accessibility → Keyboard → Sticky Keys.
A physically stuck Option, Cmd, or Ctrl key on an external keyboard - unplug it and test with the built-in one.
Modifier remapping: System Settings → Keyboard → Keyboard Shortcuts… → Modifier Keys - make sure keys aren't swapped for the current keyboard.

5. A remapper or automation app

Karabiner-Elements, BetterTouchTool, Hammerspoon, text expanders, and similar tools can rewrite keys system-wide - including with rules you set up long ago and forgot. Quit them one by one and retest. If the problem disappears, the culprit's config is where to look.

When it's actually hardware

If the same key misbehaves in every layout, in Safe Mode, and for a different user account - then it's the keyboard itself: liquid damage, debris under the key, or a failing controller. Testing with another keyboard (or the built-in one) settles it quickly.

Quick recap

Check the active layout in the menu bar first - it's the cause more often than anything else.
Verify the exact layout variant (U.S. vs. British, regular vs. PC) in System Settings → Keyboard → Text Input.
Rerun keyboard type detection for external keyboards, check Sticky Keys and modifier remapping.
Quit remapper apps like Karabiner-Elements to rule them out.
Text already typed in the wrong layout? Boomkey converts it in place with a double-tap of Shift.

If the characters are from another alphabet entirely, it is a layout problem — see how to fix text typed in the wrong layout.