Why is System Data so big on my Mac, and how to reduce it

See how much it takes

Open Apple menu > System Settings > General > Storage. System Data is the grey part of the bar. Apple describes it as files that don't belong to any more specific category.

What's in it

Caches and logs from macOS and your apps.
Local Time Machine snapshots, taken about every hour and kept for 24 hours.
App files kept outside your folders, like the media cache of video apps.
Updates downloaded and waiting to install.

Reduce it

Restart. macOS clears some caches and logs on its own when space is needed.
Clear the caches of the apps you use most, from inside each app. For video apps, see the guides below.
Remove local snapshots. In Time Machine settings set backups to Manually, wait a few minutes, then turn automatic backups back on. In Terminal, tmutil listlocalsnapshots / lists them.
Delete apps you don't use, together with the files they keep in the Library.

What not to delete

Don't delete folders in /System or /Library by hand. Apple doesn't list what in System Data is safe to remove, and deleting the wrong folder can stop apps or macOS from working.

The faster way: Caretaker

Caretaker's Disk Map shows where your space goes, then Caretaker clears what's safe: caches, logs, leftovers of removed apps and the caches of creative apps. Maintenance frees local snapshots. Caches and logs are deleted for good, since apps make them again; leftovers go to the Trash first and can be restored for 90 days.

Caretaker's Clean page: safe to clear, system junk and creative app caches, in dots.

Download free

Questions

Is System Data the same as Other?

Yes. Older versions of macOS called it Other.

Can I delete System Data?

Not as a whole. You can reduce it by clearing caches, logs, snapshots and leftovers of removed apps.