Rob Lowe Says 'Outsiders' Castmembers Are His 'Frat Brothers'

Rob Lowe 's frat class is pretty star-studded ... 'cause he says he's forever part of the "Outsiders" fraternity -- with all his costars from 1983 flick in the brotherhood.

The actor reflected on making the iconic Francis Ford Coppola movie back in the day during an interview with People ... touching specifically on turning 18 around a bunch of other guys his age -- and feeling a special kinship with them.

He says the guys on set with him were all his "homies" and "frat brothers" ... and, they brought out super compeitive sides of one another -- especially Tom Cruise who RL says was simply on another level work ethic-wise.

TOM CRUISE THROUGH THE YEARS

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In fact, Rob says it's easy to see the actors each the cast would later become ... starting with the funny-on-his-feet nature of his own character -- Soda Pop Curtis -- that helped Rob in the sitcom world and the intensity in Cruise's character that still has him doing wild stunts .

"The Outsiders" featured a bunch of other bold-faced names BTW ... including Patrick Swayze , Matt Dillon , Ralph Macchio , Emilio Estevez and Diane Lane .

Of all the guys in the cast, Lowe makes it clear he was closest in Estevez since the two grew up in the same Malibu neighborhood ... adding it was nice to have someone going through fame at the same time as him.

Sounds like the group got real tight while filming ... and, ya gotta wonder what kinda parties their unofficial frat threw!