Rocker Mod Sun was playing the last show of his current tour at The Wiltern Monday night, when he was joined onstage by MGK.

"Straight from the plane to the Mod Sun show, my friend," MGK said, addressing his pal onstage after he joined Mod for a song. He congratulated MS and the band for an "amazing" tour, and the crowd for "keeping the scene alive."

Mod responded .... "You're gonna make me cry, bro. Make some noise for my best friend in the whole f***ing world, Machine Gun Kelly!"

Then they launched into the final song of the night to the roar of the crowd.

This comes hours after Megan Fox dropped the news that she and MGK are expecting their first child together. Megan’s a mom to 3 sons -- Noah, Bodhi, and Journey -- who she shares with Brian Austin Green. MGK has a daughter, Casie, born in 2009.

Later last night, Mod Sun was seen canoodling with a mystery woman -- who was definitely not Avril Lavigne -- at the bar On The Rox in West Hollywood.

TMZ broke the story ... Mod and Avril called off their engagement in 2023 after nearly 2 years of dating. Our sources said Avril broke up with Mod and hooked up with Tyga.