Rob Riggle claims his estranged wife kept a watchful eye on him during their divorce, by secretly recording him with a hidden camera planted in his home ... and now he's suing.

According to a new lawsuit, obtained by TMZ, Rob says he started to feel as if he was being watched shortly before his wife, Tiffany, filed for divorce.

In the docs, Rob claims the couple was trying to save their marriage by "nesting" -- staying in separate homes and taking turns living with the kids in the family home -- and he got the sense Tiffany was surveilling him because she seemed to have info about his whereabouts and private messages.

Rob claims he discovered a substantial amount of money missing from a lockbox in his home office shortly after Tiffany filed for divorce ... and he says he confronted her about it because she had access to the home, but claims she denied knowing anything about the missing cash.

Not long after, Rob claims Tiffany hacked into his Apple account and downloaded his texts, contacts, photos and emails ... and then he says things got even more disturbing.

Rob claims Tiffany somehow had knowledge of private conversations he'd had in his home with his girlfriend and assistant, and Tiffany -- or other anonymous sources -- would email or text them referencing the private convos.

As a result of the alleged leaks, Rob claims he and his GF devised a misinformation scheme, planting false info to test whether any of it would leak out ... and he claims his suspicions were confirmed.

According to the suit, Rob had his home electronically swept for devices ... and he claims a hidden spy camera was discovered in a smoke detector above his desk in his home office.

Rob says the device had a memory card with over 10,000 videos with sound on it, and the device appeared to be installed in August 2020 and was transmitting or recording all video and voice activity in the home for at least 7 months.

What's more, Rob claims there is an image on the device showing Tiffany standing on a ladder and installing the spy camera, plus another showing her sitting on the floor of his home office counting cash, which he thinks is the missing money from the lockbox.

Rob's going after Tiffany for damages ... and he wants an injunction barring her from recording him and forcing her to turn over all recordings and copies.