Two actors from 'Boy Meets World' say they now have a warped view of a guest star from their show ... after the guy was later convicted on a child molestation charge.

Rider Strong and Will Friedle tackled this issue head-on alongside fellow costar Danielle Fishel on their 'Pod Meets World' podcast Monday -- where they discussed Brian Peck, an actor who was convicted on child sexual abuse charges back in the early 2000s.

At the time, he was accused of molesting an unidentified Nickelodeon child actor and ended up taking a plea deal and serving over a year behind bars. He's now set to be featured in this new documentary about the ugly underbelly of Nick that's airing in March.

Welp, as it turns out ... Rider and Will say they also knew this guy back in the day since he acted on their own show a couple times -- and even though he literally only guest-starred on two 'BMW' episodes with very minor roles, the actors now say he was deeply embedded into their lives ... and it's left them feeling creeped out and uneasy all these years later.

As they look back now, Will and Rider say his eagerness to be good pals with 'em back then -- hanging out with them a lot after work, apparently -- was wildly inappropriate and weird ... but at the time, they say they didn't suspect anything.

When allegations against him first surfaced in the early aughts -- which ultimately landed him in court -- Will and Rider say they bought Peck's story of falling victim to "jail bait" ... and even went to bat for the guy as he appeared before a judge, 'cause they believed him.

They say they now realize how twisted their relationship with this guy was ... and both express shame and regret for ever affiliating themselves with him, even as young adults.