'Big Bang Theory' star Johnny Galecki, girlfriend reveal upcoming baby's gender 

Actor Johnny Galecki and his girlfriend are expecting a baby boy.

The "Big Bang Theory" star and girlfriend Alaina Meyer revealed the news during a weekend party with the help of blue paint.

"A day I'll never forget," wrote Galecki including three heart emoji on Instagram Sunday alongside a photo of the couple, wearing painter's coveralls, kissing during outdoor festivities and covered in handprints made with various shades of blue paint.

Meyer, who posted a slightly different photo of the couple celebrating at the same party on Monday, wrote: "I can not wait to raise a baby boy with my love. He will follow in his fathers footsteps of being a generous, loving, chivalrous fellow."

The mother-to-be also posted video and still images of the celebration on her Instagram Story.

"How beautiful. Love you both," wrote Galecki's "Big Bang" co-star Kunal Nayyar in the comments section of his Instagram post.

Oscar winner Octavia Spencer (2011's "The Help," the upcoming "Ma"), commented: "Congratulations!!!!!"

"Modern Family" star Sarah Hyland wrote: "DUDE. YES!"

The baby is the first for Galecki, 44, and Meyer, who is in her early 20s.

The couple confirmed to People magazine earlier this month that they were expecting.

“We are absolutely over the moon to announce that we will soon be welcoming a little one into this crazy and wonderful world,” the two said in a joint statement.

Galecki and Meyer went public with their romance via social media in September 2018.

"The Big Bang Theory," TV's longest-running multicamera sitcom, aired its series finale earlier this month. The Emmy-nominated show ran for 12 seasons on CBS.