As mentioned by others, Descent used portal rendering all the way back in 1995. It's a simple trick that only requires an alpha mask, which even the earliest hardware supported.
But certainly the game Portal (2007) hyped using portals for rendering prior to Prey.
There are a handful of videos from E3 (97 or 98) showing off the original 3DRealms development of Prey. One of the things that didn't survive into Human Head's version on the Doom3 engine was rotating portals, or if they did they were rare that I don't remember them.
That’s right, Descent ran in mode 13h 320x200 with 256 colours and drew directly to video memory. No alpha masking involved. The source is available here https://github.com/videogamepreservation/descent
But certainly the game Portal (2007) hyped using portals for rendering prior to Prey.