To clear System Data on a Mac, shrink what's inside it, because macOS doesn't let you delete the category itself. Restart your Mac, clear app caches in your Library folder, remove big offline files from inside the apps that made them, and let macOS remove Time Machine snapshots. Free Up Space in MrFixable clears app and developer caches.

System Data is one of the categories in System Settings › General › Storage, and it can grow very large. Unlike Documents or Applications, it has no ⓘ button, so there's nothing to click and delete. The good news: most of what's in it is temporary, and some of it is safe to clear.

What is System Data on a Mac?

System Data is everything on your Mac that doesn't fit one of the other Storage categories. Apple lists logs, caches, virtual memory files, temporary files, fonts, app support files and plug-ins. macOS manages it, so Storage settings don't let you open or delete it directly. In practice it's mostly:

Caches: files apps save to work faster and make again when they're gone.
Temporary and virtual memory files: files macOS uses while it's running, cleared when you restart.
App data and snapshots: data apps keep in your Library folder, and Time Machine's local snapshots.

How do I see how much System Data I have?

Open System Settings › General › Storage and wait for the bar at the top to finish calculating. System Data is listed below it with its size, along with categories like Applications, Documents and Photos. It has no ⓘ button, because macOS manages what's in it, so you can only see the total.

Choose Apple menu › System Settings.
Click General in the sidebar, then click Storage.
Wait for the bar at the top to finish calculating, then find System Data in the list below it, with its size.

The size can change a lot from day to day. That's normal: caches and snapshots come and go.

How do I clear System Data on a Mac?

Clear the parts of it that are safe to remove, starting with the easiest. A restart clears temporary files, clearing app caches often frees the most, and Time Machine snapshots go away on their own. Work through these steps in order, and check System Settings › General › Storage again after each one.

Restart your Mac. Choose Apple menu › Restart. This clears temporary files and the virtual memory files macOS uses as extra memory. If you rarely restart, do this first.
Clear app caches. Quit your apps, then in Finder choose Go › Go to Folder… (⇧⌘G), type ~/Library/Caches and press Return. Move the folders of apps you recognize to the Trash and leave folders whose names start with com.apple alone. Step-by-step: how to clear the cache on a Mac.
Remove offline files inside the apps that made them. Music, video, maps and podcast apps can keep gigabytes of downloads for offline use in their data folders. Remove those downloads from each app's own settings, not by deleting folders in Finder.
Uninstall apps you no longer use, with their leftovers. An app's settings, caches and saved data stay in your Library folder after you drag the app to the Trash. See how to uninstall apps on a Mac completely.
Let Time Machine clear its snapshots. If you use Time Machine, your Mac keeps local snapshots for about 24 hours and removes them sooner when it needs the space. To clear them now, Apple suggests turning off automatic backups in System Settings › General › Time Machine for a few minutes, then turning them back on.
Empty the Trash. Choose Finder › Empty Trash. Nothing you moved to the Trash frees any space until you do.

What should I not delete?

Don't delete anything in /System, don't empty the /Library/Caches folder at the top of your disk, and don't delete the ~/Library/Caches folder itself, only folders inside it. Be wary of Terminal commands that promise to wipe System Data in one go. Time Machine snapshots don't need deleting by hand either: your Mac counts their space as available.

Clear the safe parts in one step

Free Up Space in MrFixable lists the big hidden files that are safe to remove, each with its size, and shows how much space is free on your Mac:

App caches, with the biggest ones named. macOS's own caches and the caches of apps that are open are skipped.
Xcode build files, old iPhone and Watch debug files (the newest one for each kind of device is kept), and the npm and Gradle download caches, if you have them.
Downloads older than 30 days and the Trash, unticked to begin with, so you decide.

Tick what to clean and click Free Up Space. Caches and build files are deleted for good, because apps and tools make them again when they need them. Old downloads go to the Trash instead, so you can still get them back.

Free Up Space cleans only the things on its list. It won't clear everything macOS counts as System Data, and nothing can safely do that.

Questions

How do I delete Mac System Data?

You can't delete System Data as a whole: Storage settings have no ⓘ button for it, because macOS manages it. Instead, restart your Mac, clear app caches in ~/Library/Caches, remove offline downloads inside the apps that made them, uninstall unused apps with their leftovers and empty the Trash. System Data shrinks as you go.

How do I reduce System Data on a Mac?

Restart your Mac first: that clears temporary and virtual memory files. Then clear app caches, delete apps you don't use together with their Library files, and empty the Trash. If you use Time Machine, give your Mac a day: it removes local snapshots after about 24 hours, or sooner when it needs the space.

Is it safe to clear System Data on a Mac?

Parts of it, yes. Restarting, clearing app caches, emptying the Trash and removing offline downloads inside apps are all safe: apps make their caches again when needed. Deleting system folders or running Terminal commands that wipe System Data is not safe, and can stop apps or macOS from working properly.

How to clear System Data on a Mac, and what's safe to remove