Wanderlust lovers!

Jennifer Aniston hasn't had a serious boyfriend since John Mayer in 2009 -- but that seems to have finally changed with Justin Theroux, her Wanderlust costar-turned-beau.

An insider tells Us Weekly that Aniston, 42, and Theroux, 39, have been spending quality time "for months" and have been inseparable more recently -- having a sleepover after a May 18 dinner with pals in Hollywood, then a cozy weekend holed up at her home.

Get more details on Aniston's hot new fling in this week's Hot Stuff video, as told by Senior Editor Ian Drew plus special guest Nancee Bingenheimer from VH1's reality show Saddle Ranch. Also in the roundup: Reese Witherspoon gets "serious" about a new hobby, Justin Timberlake's flings Mila Kunis and Olivia Wilde hang out, and more!