Kathryn Bridget Moynahan (born April 28, 1971) is an American actress and former model best known for her role as Erin Reagan in the police drama Blue Bloods. She graduated from Longmeadow High School in Massachusetts in 1989 and began pursuing a career in modeling. She appeared in department-store catalogs and magazines, and after doing television commercials, began taking acting lessons. She made her television debut in a guest appearance in the comedy series Sex and the City in 1999, where she later had a recurring role as Natasha.

She made her feature-film debut in Coyote Ugly (2000). She had supporting roles in Serendipity (2001); The Sum of All Fears (2002); The Recruit (2003); I, Robot (2004); Lord of War (2005); Grey Matters (2006); Prey (2007); Noise (2007); Ramona and Beezus (2010); John Wick (2014); The Journey Home (2014) and John Wick: Chapter 2 (2017).

She starred in the ABC television series Six Degrees, which premiered in September 2006, and was taken off the schedule after just eight episodes. Since September 2010, she has starred as an assistant district attorney in the CBS drama Blue Bloods.