Lukas Haas and Anderson .Paak are in the Christmas spirit ... because Lukas got on the mic at Anderson's karaoke bar and belted out his new holiday song ... and we have it all on video.

Check out this video, obtained by TMZ, from inside Anderson's music bar in West Hollywood, Andy's, where Lukas showed up on Christmas eve for a live performance of his new single, "Christmas Cheers."

The song was originally written for Lukas' character on the HBO hit "Righteous Gemstones" ... and as you can tell, it's not your traditional Christmas story.

Anderson's karaoke spot was having an ugly Christmas sweater night too ... and he and Lukas played along.

Folks also got treated to some karaoke from Anderson ... he sang "The Reason" from Hoobastank.

When Anderson was talking to the crowd, he mentioned he was single ... which is pretty interesting, considering he was getting close with Mariah Carey over the past year.

FYI ... Tuesday is the exclusive Casablanca karaoke party at Andy’s where different VIP’s come and pop-up on stage.

Looks like a merry time was had by all ... and Anderson even did some crowd surfing!!!