The short answer
Hidden photos are part of your Photos library, and iCloud Photos syncs the library. A hidden photo is uploaded to iCloud, appears in the Hidden album on your iPad and Mac, and is included when the library is backed up. Hiding it keeps it out of view. It does not keep it out of iCloud.
What syncs
- Hidden photos and videos, to the Hidden album on every device with iCloud Photos on.
- Recently Deleted, for its 30-day window.
- Edits, favourites and album membership.
What does not sync
- Photos in a third-party vault app that keeps its own storage. Those are the app's files, not the Photos library, and iCloud Photos never sees them.
- Photos you delete from Recently Deleted. Once gone from every device, they are gone from iCloud Photos.
Where a hidden photo can show up
- The Hidden album on a shared family iPad or Mac signed in to your Apple Account.
- Anywhere your Apple Account is signed in with iCloud Photos on, including a device you forgot about.
- A device backup, which restores the whole library including hidden items.
How to keep a photo out of iCloud
Move it into a vault app that stores files itself, then delete the original from Photos and from Recently Deleted. Kryptis encrypts the file on the phone with AES-256-GCM and keeps it in its own storage. If you want a backup, Kryptis makes an encrypted file you save to iCloud Drive yourself; iCloud stores the encrypted file, not a photo it can read. How Kryptis backup works.
If you want iCloud Photos but not on one device
Turn off iCloud Photos on that device only, in Settings, then your name, iCloud, Photos. The library stops syncing to that device. Hidden photos already downloaded there stay until removed.
