The singer was spotted out with Wilmer Valderrama on Saturday night, surprising onlookers by getting close at L.A. hot spot Wonderland.
Lavigne, who filed for divorce from Whibley last month, arrived with Valderrama at midnight.
"Avril had her hand on his knee at one point," says an onlooker. "They looked super cozy together." The couple stayed close until leaving just after 2 a.m.
"There were others at the table but you could tell they only had eyes for each other," the onlooker continues. "They were whispering close the entire night, laughing and having a lot of fun."
A source close to Lavigne tells PEOPLE that the couple has been getting close for the past few weeks. "They've been friends for a long time and have worked together before," says the source.
Meanwhile Whibley, 29, has been spotted out in Las Vegas and L.A. with model/designer Hanna Beth Merjos. "He's enjoying being single for the first time in many years and he's been spending a lot of time working on a new album," says a pal of the Sum 41 rocker.
Reps for Lavigne and Valderrama couldn't be reached for comment.
Reporting by JENNIFER GARCIA