"Anna has been dating Bill quietly for over a year," says the source. "They met years ago. She's hosted Saturday Night Live and they've done a movie together, but they got together well after the movie." (Kendrick hosted SNL in 2014, though Hader left the cast in 2013.)
"They are both very private people, and with the pandemic it was easy to keep it quiet," adds the source. "They're both hysterical so they must keep each other laughing all of the time. She's really, really happy."
Reps for both actors declined to comment for PEOPLE.
The Oscar-nominated Pitch Perfect actress, 36, and Hader, 43, costarred in the 2019 Disney+ Christmas movie Noelle together.
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Bilson and Hader had starred together in the 2013 romantic comedy The To Do List. Maggie Carey, that film's director, was married to Hader from 2006 to 2018. They share three daughters: Hayley Clementine, 7, Harper, 9, and Hannah Kathryn, 12.
Back in 2020, Kendrick opened up about her dating history while discussing her HBO Max series Love Life. She told The Sydney Morning Herald that she is "just so happy that I'm clearer now about what I will accept from people in my life."
"The real arc that we wanted was to show how we learn from each relationship. Even though our relationships end, it doesn't mean they are complete failures," explained Kendrick at the time. "People come into your life and you grow and they change you, and what you learn from them might be really positive or it might be negative."