Lyndsy Marie Fonseca (born January 7, 1987) is an American actress, best known for her roles as Colleen Carlton on the CBS soap opera The Young and the Restless, and Alexandra "Alex" Udinov on The CW show Nikita. She was discovered by a talent manager in San Francisco. She then relocated to Los Angeles for her first pilot season at age 13, in the middle of the school year. The following year, she landed her breakthrough role, playing Colleen Carlton on The Young and the Restless with a three-year contract. In she had a recurring role as Ted Mosby's future daughter on How I Met Your Mother and as Donna on HBO's Big Love. She starred in the 2005 Hallmark TV film Ordinary Miracles, as a 16-year-old juvenile delinquent who goes to live with a judge (Jaclyn Smith).In fall 2007, she played Dawn in the film Remember the Daze. She joined the cast of ABC's Desperate Housewives as the daughter of Katherine Mayfair (Dana Delany), a character moving onto Wisteria Lane.

In 2008, she was nominated at the Screen Actors Guild Awards for Outstanding Performance by an Ensemble in a Comedy Series for Desperate Housewives (2004), shared with the cast.

She played Katie Deauxma in the 2010 superhero film Kick-Ass, which also featured her Nikita co-star Xander Berkeley in the cast. She appeared in three more films in 2010, including Hot Tub Time Machine, and she was cast as new recruit Alex on the CW's Nikita. She was listed at #62 in the 2010 Maxim Hot 100 and #89 in the 2011 list.