"We need a general election and we need one now. We have at this moment a one-off chance to get this done... before the detailed talks begin," May said, despite previously denying that she would do so.
Speaking outside her Downing Street residence in London, May warned that "division in Westminster will risk our ability to make a success of Brexit".
She said parliament would be asked to vote Wednesday to decide on whether or not to hold an election.
The dramatic announcement comes after months of tumult in British politics following the Brexit vote.
A round of opinion polls over the Easter weekend also showed her Conservative Party far ahead of the main opposition Labour Party.
The Conservatives polled at between 38 percent and 46 percent, with Labour at 23 percent to 29 percent, according to the polls by YouGov, ComRes and Opinium.
The poll lead had prompted many senior Conservatives to call for an election, particularly as May will need a strong parliamentary majority as she seeks to negotiate Britain's exit from the European Union.
The Conservatives currently have a working majority of just 17 from the last election in 2015 and some of their MPs have indicated they could vote against the government on key aspects of Brexit legislation.
EU leaders except May are set to hold a summit on April 29 where they will agree on the strategy for negotiating Britain's expected departure in 2019.
The negotiations themselves are not expected to start until May or June at the earliest. The European Commission has said it wants the exit talks to be concluded by October 2018 at the latest.
Britain's next election was due to have been held in 2020 -- a date enshrined in legislation according to which elections have to be held every five years in May.
But the law can be overruled if two-thirds of lawmakers in the British parliament vote in favour of early elections -- something that Labour leader Jeremy Corbyn has previously indicated he would do.
Corbyn, a veteran socialist with support on the left of the party, won the Labour leadership in September 2015 after the party's defeat in that year's election.
- High approval rating -
Corbyn, 67, enjoys grassroots support from left-wingers but is opposed by most of the party's more centrist lawmakers, who say that Labour under his leadership is not appealing to the middle classes.
May in contrast has scored consistently well in terms of personal popularity and polls have shown approval of her handling of the run-up to Brexit negotiations.
When asked who they thought would be the best prime minister, 50 percent of respondents in the YouGov poll said Theresa May and only 14 percent Corbyn.
May came to power in July 2016 after her predecessor David Cameron resigned following the shock Brexit referendum vote in June for which he had campaigned for Britain to stay in the European Union.
The 60-year-old vicar's daughter is Britain's second prime minister after Margaret Thatcher and many commentators have drawn comparisons to the steely determination of the "Iron Lady".
May worked in finance, including at the Bank of England, before being elected as MP for the London commuter town of Maidenhead in 1997.
As Conservative chairwoman in 2002, she made waves by suggesting the Tories were seen as "the nasty party" and needed to overhaul their image — something that they did under Cameron's leadership.
When the Conservatives won the 2010 general election, May was named home secretary, the hardest job in government which has wrecked a string of other political careers.