The image confirms to audiences and fans of the ABC sitcom that Galecki is coming back to play David Healy.
"21 YEARS LATER...! Apart from being born, possibly the most surreal experience of my life. Deep thanks to my Big Bang Theory family for knowing the importance of visiting one's roots and loaning me out for a quick minute," Galecki wrote in the caption.
Earlier this week, Roseanne Barr confirmed that Galecki will be taking part in the revival. The 42-year-old played David, the husband of Sara Gilbert's Darlene. Coincidentally, both Gilbert and Laurie Metcalf, who plays Roseanne's sister, also appeared on Big Bang.
"We have got our Johnny Galecki back on 'Roseanne' this episode — what a joy to work with such a deeply empathetic actor," Barr tweeted. "Very moving 4 all of us. [JOY]."
The new season of Roseanne will debut on ABC in early 2018, with at least nine new episodes. Since production began, Barr and the show's social media team have been sharing on-set photos with fans, showing the iconic Conner living room recreated.
The original Roseanne lasted nine seasons, from 1988 to 1997. Galecki joined the show during the fourth season. He plays Leonard on The Big Bang Theory and earned a Golden Globe and Emmy nomination for the role.