John Mark Galecki (born April 30, 1975) is an American actor. He is known for playing Leonard Hofstadter in the CBS sitcom The Big Bang Theory since 2007 and David Healy in the ABC sitcom Roseanne from 1992 to 1997 and briefly reprised the role during its 2018 revival. Galecki also appeared in the films National Lampoon's Christmas Vacation (1989), Prancer (1989), Suicide Kings (1997), I Know What You Did Last Summer (1997), Bookies (2003), In Time (2011), and Rings (2017).

Galecki is one of the highest paid television actors in the world, with his role in The Big Bang Theory currently earning him approximately US$900,000 per episode between 2017 and 2019.