Martha Maria Yeardley Smith (/ˈjɑːrdli/ YARD-lee; born July 3, 1964) is an American actress. She stars as the voice of Lisa Simpson on the animated television series The Simpsons.

Smith began acting in 1982 after graduating from drama school. She moved to New York City in 1984, where she appeared in the Broadway production of Tom Stoppard's The Real Thing. She made her film debut in 1985's Heaven Help Us, followed by roles in The Legend of Billie Jean and Maximum Overdrive. She moved to Los Angeles in 1986 and took a recurring role in the television series Brothers.

In 1987, Smith auditioned for the Simpsons shorts on The Tracey Ullman Show. Smith intended to audition for the role of Bart Simpson, but the casting director felt her voice was too high, and she was cast as Bart's sister Lisa. In 1989, the shorts were spun off into their own half-hour show, The Simpsons. For her work on The Simpsons, Smith received a Primetime Emmy Award for Outstanding Voice-Over Performance in 1992.

Alongside The Simpsons, Smith appeared in the sitcom Herman's Head as Louise, and had recurring appearances as Marlene on Dharma & Greg and as Penny in two episodes of Dead Like Me. She has appeared in several films, including City Slickers, Just Write, Toys, and As Good as It Gets. In 2004, Smith performed an off-Broadway one-woman show entitled More at the Union Square Theatre in New York City. Aside from The Simpsons, Smith has recorded few voice-over parts, only commercials and the film We're Back! A Dinosaur's Story. Smith starred in and served as executive producer for the independent romantic comedy Waiting For Ophelia, which had its world premiere at the Phoenix Film Festival in April 2009.

Smith was born on July 3, 1964, in Paris, France. Her father, Joseph Smith, worked for United Press International in Paris and moved to Washington, D.C., in 1966, where he became The Washington Post's first official obituary editor. Her mother, Martha Mayor, was a paper conservator for the Freer and Sackler Galleries at the Smithsonian Institution. Smith's parents later divorced. Her maternal grandfather was art historian A. Hyatt Mayor, and among her great-grandparents were marine biologist and zoologist Alpheus Hyatt and artist and sculptor Harriet Randolph Hyatt Mayor. She is also the paternal niece of political scientist, historian, and Latin American studies specialist Peter H. Smith. Smith has labeled her family "upper crust and reserved". As a child, Smith was often teased because of her unusual voice. Smith has stated: "I've sounded pretty much the same way since I was six. Maybe [my voice is] a little deeper now." She made her acting debut in a sixth-grade play.

Smith became a professional actress in 1982. After appearances in a number of school plays, she joined the local Arena Stage theater group on an apprenticeship, featuring in their production of Peter Pan. She went on to star in several other plays in Washington. She moved to New York City in 1984 and appeared in the Broadway production of Tom Stoppard's play The Real Thing alongside Jeremy Irons and Glenn Close.

Smith's first film role came in Heaven Help Us (1985). She then played Putter in The Legend of Billie Jean (also 1985). The film was a box office bomb and critically panned, although Smith "thought it would be the movie that launched my career. And then it was out at the box office about 10 days before it died." When filming was over, she rejoined The Real Thing before being out of work for six months. Smith worried her career was over. However, the following year, she played Connie in Stephen King's Maximum Overdrive (1986), noting it was "truly a dreadful film, but I had a great part in it."

Smith moved to Los Angeles in 1986 on the "semi-promise" of a part in a TV film. After the audition, the role was given to another actress. Smith realized "that people don't mean what they say. It's not malicious. They just don't realize how much impact they have on an impressionable actor – and all actors are impressionable." From then on, she decided to "just sort of build a wall around myself", to cope with the disappointment of not getting a part. In Los Angeles, Smith appeared in theatrical productions of Living on Salvation Street, for which she was paid $14 for each performance, Boys and Girls/Men and Women, and How the Other Half Loves, and played the recurring role of Louella Waters on the Showtime series Brothers. She appeared in the films The Legend of Billie Jean and Ginger Ale Afternoon (1989) as "trailer-park girls". She later spoke of her regrets of appearing in the latter in her one-woman show More.