Rob Lowe is looking back on a short romance he had with Demi Moore ... revealing they briefly hooked up early on in their Hollywood careers.

Rob told Kelly Ripa he "briefly had a thing" with Demi in the '80s during their time in the Brat Pack ... admitting to the hookup on Kelly's podcast, "Let's Talk Off Camera."

The way Rob tells it ... he and Demi banged at some point in the mid-'80s, before she got engaged to Emilio Estevez, another member of the Brat Pack.

Rob joked the hookup was part of a "big, big sex orgy" among the cast of the 1985 movie "St. Elmo's Fire."

But that wasn't the only movie in which Rob and Demi were paired up on set ... they also starred alongside one another in another flick, 1986's "About Last Night."

Looking back, Rob explains ... "Anytime you put young 20-something men and women together, hookups are inevitable."