Schwarzenegger cops to the sexual tryst in his new book, saying it went down while the duo was filming "Red Sonja" together back in 1985.
At the time of the affair, Arnold was already knee deep into his relationship with Shriver -- and even though they weren't married, the couple was living together.
If you recall ... Nielsen blew the whistle on the affair last year in her own published memoirs. Arnold never confirmed it -- until now.
Nielsen said in her book that she didn't know Arnold was involved with another woman during their fling.