Audrina Patridge is backing Spencer Pratt's mayoral run ... saying she knows he'll deliver on his promises.

We caught up with "The Hills" star at Calamigos Resort in Malibu Friday night ... and, we asked her about her former costar's run for L.A. mayor.

Patridge says, once she realized he wasn't joking, she was downright impressed with Spencer's campaign ... adding she knows firsthand how determined he is once he puts his mind to something.

She tells us she thinks he will live up to his campaign promises ... and, though she can't vote for him because she doesn't live in Los Angeles, she says she's certainly rooting for him.

As you know ... Spencer's taking on incumbent L.A. Mayor Karen Bass -- and, he's confident he'll win this election.

While some -- like Patridge and and Lakers Governor Jeanie Buss -- have shown support for Spencer, others are less sure.

Pratt's sister, Stephanie, warned people not to vote for him ... and, his former costar Lauren Conrad responded awkwardly when asked about the run.

To each their own ... but, count Patridge among those who are Team Spencer!