The 39-year-old Big Bang Theory star and 30-year-old Pan Am actress were together for over two years, and a source tells Us Weekly the split was amicable.
"Johnny and Kelli are still best friends," the insider says.
Galecki previously dated co-star Kaley Cuoco from 2007 to 2009. He and Garner were spotted holding hands and kissing on a beach in Mexico in January, days after they attended Cuoco's wedding to tennis player Ryan Sweeting.
Galecki, Cuoco and Jim Parsons signed new, three-year contracts with The Big Bang Theory earlier this month. The trio portray Leonard, Penny and Sheldon, respectively, and will reportedly earn $1 million per episode. The Big Bang Theory returns to CBS on September 22.