The iconic actress and model shared the flashing allegation in an excerpt from her book, 'Love, Pamela,' which Variety obtained. In it, PA details an event that she claims happened in 1991 on TA's sitcom at the time, 'Home Improvement' ... when she was 23 years old.

She writes, "On the first day of filming, I walked out of my dressing room, and Tim was in the hallway in his robe. He opened his robe and flashed me quickly — completely naked underneath."

Pam adds, "He said it was only fair, because he had seen me naked. Now we’re even. I laughed uncomfortably." Tim, for his part, tells TMZ ... "No, it never happened. I would never do such a thing." So, he's straight up saying it didn't happen ... period.

Now, if this happened in '91 -- as she claims -- then it would've presumably been during the first season of 'HI' ... which started airing that same year. Pam was on a lot of episodes in Season 1 as 'Tool Time' girl, Lisa, and even appeared in the pilot.

Pam went on to costar in the second season of 'Home Improvement,' and made a cameo as her original character in season 6. Her other show, "Baywatch," was taking off in the early '90s ... as was the rest of her career.

This is one of the biggest bombshells that has surfaced from Pam's upcoming memoir, which is hitting shelves at the end of the month. Other headlines have touched on the fact that she believes Tommy Lee was her one true love, and that the sex tape ruined their marriage.