'Zoey 101's Matthew Underwood Reveals Sexual Assault Story Post-'Quiet on Set'

Matthew Underwood is speaking up on 'Quiet on Set' -- and while he says he himself was never mistreated while working as a child star ... he claims he's been harmed in the industry.

The "Zoey 101" star posted a lengthy statement Friday on his social accounts that addressed why he hadn't said anything publicly about the new doc -- which chronicles different allegations levied by former child stars with the network -- and he explains his rationale.

MU writes, "We all want to live in a better world. That requires all of us to treat each other with empathy and not make assumptions about people’s personal experiences."

He adds, "We’re all feeling pain right now, and I can understand if you are angry, but harassing people who maintain their privacy and wishing death upon their families is no way to make this world better and safer for all of us. #QuietOnSet."

In the post itself, Matt reveals he was molested by a friend's stepfather at the age of 12 -- and claims he was sexually harassed and assaulted by a former agent at age 19. Matt says this caused him to leave acting behind, and leave Los Angeles entirely.

In terms of what's been alleged in the doc itself, Matt says it simply wasn't his experience -- noting he enjoyed his time working for Nickelodeon and with Dan Schneider.

His larger point, though, is that people who watched the doc have been hounding him, his family and his peers who haven't said anything ... something we've seen happen with others in this saga, including Josh Peck -- whose social media comments were flooded with hate.