Millionaire environmentalist Zac Goldsmith won the west London seat of Richmond Park for the Conservatives.
Mr Goldsmith beat sitting Liberal Democrat MP Susan Kramer to take the seat, with 29,461 votes to her 25,370.
He thanked his opponent and his team, adding: "This is the end of a long and hard campaign."
Mr Goldsmith, who advises Tory leader David Cameron on green issues, warned: "It's the beginning now of a much harder campaign."
He faced controversy in 2009 over his non-domiciled tax status, which threatened to embarrass the Tories after they pledged to impose a £25,000 annual levy on non-doms.
There was a 7% swing from the Lib Dems to the Conservatives in Richmond Park.