A source tells ET that the 36-year-old actress and 43-year-old SNL alum have been "dating for a while" now and are both "very happy" in the relationship.
"Anna and Bill have been dating for a while. They really like each other and share the same sense of humor," the source says. "They're both very happy in the relationship and it shows. They have a natural connection."
The pair starred in the 2019 Disney+ Christmas movie, Noelle, together, but it's unclear when they officially began dating.
Hader was most recently linked to The Hart of Dixie star, Rachel Bilson, but the pair called off their relationship in July 2020, just six months after making their red carpet debut at the 2020 Golden Globes.
The two actors co-starred in The To Do List in 2013, which was written and directed by Hader's then-wife, Maggie Carey. Hader and Carey split in 2017, and share three daughters together: Hannah, 12, Harper, 9, and Hayley, 7.
Kendrick, on the other hand, had been in a relationship with British cinematographer Ben Richardson since February 2014, which she kept private. She and Richardson were last photographed together in 2019.