In an unexpected statement at Downing Street, May said she was starting the process of calling a vote on June 8, less than halfway through the government's term.
May said that a new mandate would strengthen her hand in negotiations over Britain's departure from the European Union, and said there was a division on Brexit among parties at Westminster.
"There should be unity here in Westminster, but instead there is division. The country is coming together, but Westminster is not," she said.
She called on voters to throw their support behind her Conservative Party, adding that "every vote for the Conservatives will make me stronger" in Brexit talks.
May confirmed she would be seeking parliament's approval to call the early vote. Under legislation introduced by her predecessor, David Cameron, an early election needs at least two-thirds of the vote in parliament's lower House of Commons.
The decision was announced at a low point for the main opposition Labour Party's history -- approval polls show that the ruling Conservative Party is currently around twice as popular as Labour, and May's popularity is more than three times Labour leader Jeremy Corbyn.
The early election is likely to translate to more seats for the Conservative Party and would give May the Brexit mandate she is looking for.
The Conservatives already hold the balance of power in the House, but only by a slim majority, with 330 of the 650 seats. Brexit has been a divisive subject in the House that has penetrated party lines.
May came to power last July in the wake of the British vote to leave the EU, which led to Cameron's resignation.