The deal will allow May to pass legislation with the backing of the DUP in parliament, and stay in power as she attempts to negotiate Britain's exit from the European Union.
British Prime Minister Theresa May struck a deal on Monday to prop up her minority government with the support of a small Northern Irish Protestant party.
After May lost her majority in parliament on June 8 with a failed gamble on a snap election, she tried to secure the backing of the small Democratic Unionist Party (DUP) and its 10 lawmakers, though talks had dragged on for more than two weeks.
May and DUP leader Arlene Foster presided at the signing of a deal at Downing Street on Monday.
They smiled and joked as negotiators from both sides, the DUP's Jeffrey Donaldson and the Conservatives' Gavin Williamson signed the deal.
The details of the deal were not immediately released.
TRT World's Simon McGregor-Wood reports from London.
The deal doesn't guarantee May's future
A deal would allow May to pass legislation with the backing of the DUP in the 650-seat parliament, and stay in power as she attempts to negotiate Britain's exit from the European Union.
But May's position remains insecure. Her Brexit strategy is under scrutiny and her future as prime minister is the subject of public debate.
Some senior Conservatives have voiced unease at a deal with the DUP, saying it could put at risk the 1998 peace settlement in Northern Ireland, known as the Good Friday Agreement. Foster said a deal with May could help drive a second deal on power sharing in the province, possibly this week.
Northern Ireland has been in crisis since Sinn Fein pulled out of government in January, prompting an election in March and a series of missed deadlines to restore the compulsory coalition between Irish Catholic nationalists and pro-British Protestant unionists.
"I think that this agreement will bring the prospects of doing a deal at (the Belfast parliament) Stormont closer because this will have a positive impact in relation to Northern Ireland," Foster said.
"I very much hope that this week we will be able to conclude on two agreements."
The latest deadline set by the British government for the parties in Northern Ireland to reach an agreement is Thursday. Sinn Fein said last week that "time was running out" given the lack of knowledge about the impact of any Conservative-DUP deal.