Cuoco, who plays Penny in the sitcom, told CBS Watch! magazine that she and Galecki, who plays on-screen love interest Leonard, split "before the holidays" last year due to the pressure in keeping their relationship out of the public eye.
She said: "It was such a huge part of my life and no-one knew about it. It was a wonderful relationship but we never spoke a word about it and never went anywhere together.
"We were so protective of ourselves and the show and didn't want anything to ruin that. But that also made it sad, too. That's not the kind of relationship I want - I don't want to be hiding."
The 24-year-old added that she and Galecki are still friends, and were able to stop their break-up from affecting their working relationship.
"When you're dealing with two professionals who have been working since they were both 4 - and who kept their lives extremely private - we didn't want anything other than that," she explained.