Modest Mouse guitarist Simon O'Connor is sitting out the band's next tour after allegedly sliding into an underage fan's DMs with an explicit image ... then telling her a night of partying -- and her bikini pics -- helped spark the exchange.

The fan shared screenshots on Reddit showing messages O'Connor allegedly sent before her 17th birthday. She claims her age was clearly displayed on her Instagram bio at the time.

Take a look through the DMs ... the alleged exchange starts with O'Connor sending a meme and a sexually explicit image, prompting the teen to ask whether he'd been hacked.

O'Connor allegedly said he hadn't been hacked and blamed the messages on a "party night" ... before suggesting the fan's bikini photos had also caught his attention.

The guitarist -- who joined the band in 2021 and played on their latest album, "An Eraser and a Maze"-- later posted his own lengthy explanation on the Modest Mouse subreddit ... saying he mistook the teen for someone who had repeatedly harassed and pranked him through fake accounts.

O'Connor admitted he used "poor judgement" when he fired off what he called a tasteless meme and a "Simon calls your bluff" sticker ... but said he unsent the messages and ended the conversation once he realized he had the wrong person.

He apologized to the fan ... saying she didn't deserve to be put in that position, and said he strongly opposes any sexual behavior involving minors.

Modest Mouse has since announced O'Connor won't join its upcoming North American tour, which kicks off Tuesday in Ojai, California ... Seth Jabour of Les Savy Fav will step in on guitar in his place.

The band didn't clarify whether O'Connor is gone for good ... but for now, he's definitely not floating on with the tour.

We've reached out to the band's reps ... so far, no word back.