The "Summer of '69" singer also spills on the "arty" dick pic he sent Elton John for the rock star's 42nd birthday.
The musician stopped by "Watch What Happens Live" Monday night and played some "Plead the Fifth," in which Andy Cohen's guests answer three questions, but can skip one. For his second question, Cohen asked the '80s and '90s hitmaker about a rumored relationship with Prince Harry and Prince William's mother.
"There are many rumors that you and Princess Diana were once romantically involved," Cohen asked. "Her butler said that he used to sneak you into Kensington Palace. How would you characterize your relationship with Princess Diana?"
"Great friends," Adams said unhestitatingly. "And she didn't sneak me in, I would just roll up."
"Friends with benefits?" Cohen asked, prying for more information.
"She was just, we were good friends," Adams responded. We really wish Cohen pried for more information, but we'll take what we got.
Adams was also asked about "a dick pic" he gave to Elton John -- in a frame -- and he was totally game to talk about it.
"That was the first and only time," he said. "It was an arty picture, by the way."