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.”