Lyndsy Marie Fonseca (born January 7, 1987) is an American actress. She began her career by appearing as Colleen Carlton on the CBS daytime soap opera The Young and the Restless, on which she starred between 2001 and 2005. Thereafter, she had a series of other recurring roles, including Penny Mosby on the CBS sitcom How I Met Your Mother, Donna on HBO's Big Love, and Dylan Mayfair on the fourth season of the ABC television series Desperate Housewives.

From 2010 to 2013, Fonseca starred as Alex Udinov on The CW's Nikita, and from 2015 to 2016 she played Angie Martinelli on ABC's Agent Carter. She has also appeared in a variety of film roles, including Jenny in Hot Tub Time Machine (2010) and Katie Deauxma in Kick-Ass (2010) and its 2013 sequel. She has also been featured in a number of Hallmark Channel movies.