Amanda Seyfried might have felt 'Les Misérables' when she accidentally exposed herself to Glen Powell ... though she does admit the actor was a total gentleman about it.

The actress told the story in an interview with People to promote her new Peacock show "Long Bright River" ... explaining that when she was at the 2023 Critics Choice Awards, she wore a flimsy vintage Dior Haute Couture gown.

Check out the pics ... the ensemble has a plunging neckline, and a sheer material that's covering her breasts -- which Amanda says didn't hold up for the whole night.

In fact, Amanda says that she felt the dress breaking apart while doing interviews on the red carpet ... so, from the beginning, it seemed like the gown was doomed to break open

Amanda explains later in the evening she and Glen were seated at the same table ... and, when she raised her arms to tell a joke, the dress slipped down -- revealing every bit of her bosom to the star.

Instead of gawking or making a creepy comment, Seyfried says Powell acted like a true gentleman ... moving to cover her up and not even taking a peek -- at least, Seyfried says she doesn't think he caught a glimpse of her goods.

Seyfried won Best Actress in a Limited Series or Movie Made For Television for her role as Elizabeth Holmes in "The Dropout" BTW ... and, she borrowed a friend's jacket to cover up her boobs since the dress couldn't do the job.

'Dropout' sounds like a pretty apt description of Amanda's night!!!