The 78-year-old actress said it was "Thriller" hitmaker's idea, reported People magazine.
"Of course," she responded when Ellen DeGeneres asked if she'd swam nude with Jackson.
"Here's how it happened; he was visiting me when we were shooting On Golden Pond and I had a little cabin on the lake.
"He stayed with me for about eight days and one night it was a full moon and he said lets go swimming. It was his idea!"
Fonda said the second time she went swimming without her clothes was with Hollywood legend Greta Garbo.
"Then one day Garbo came and she said 'You wanna go swimming?' She came out with a robe and one of those serious, white, rubber caps... And dove in.