Under the agreement, about £1bn in "new money" has been earmarked for Northern Ireland, as well as greater flexibility over £500m already allocated to the country.
A large portion of the cash - £400m - is going to be spent on infrastructure, with "significant" investment also planned in health and education.
In exchange, the Conservatives will now receive support from the DUP's 10 MPs on key votes in the Commons - including those relating to the Queen's Speech, the Budget, national security and Brexit.
:: How Northern Ireland's extra £1bn will be spent
DUP leader Arlene Foster has confirmed there will now be no changes to the triple lock on pensions anywhere in the UK following the deal - with the Tories also abandoning plans to means-test winter fuel payments.
The agreement was signed inside Downing Street by Tory chief whip Gavin Williamson and the DUP's Sir Jeffrey Donaldson while the party leaders watched on.
Once the deal was reached, the Prime Minister said: "I welcome this agreement which will enable us to work together in the interest of the whole United Kingdom, give us the certainty we require as we embark on our departure from the European Union, and help us build a stronger and fairer society at home.
"The agreement we have come to is a very, very good one, and we look forward to working with you."
News that the deal has been confirmed immediately sparked calls for other parts of the UK to receive a similar boost in funding - and questions were also raised about whether the pact was legally compatible with the Government's obligations under the Good Friday Agreement.
Labour leader Jeremy Corbyn said the agreement was not in the national interest, adding: "The Government must immediately answer two questions. Where is the money for the Tory-DUP deal coming from?
"And, will all parts of the UK receive the much needed additional funding that Northern Ireland will get as part of the deal?"
Welsh First Minister Carwyn Jones described the £1bn injection as a "straight bung to keep a weak Prime Minister and a faltering Government in office".
Plaid Cymru went one further, and said Wales should receive a £1.7bn boost in funding given it has a larger population.
Although Sinn Fein said the deal could help public services, the party warned it is providing Theresa May with a "blank cheque" for a Brexit which threatens peace in Northern Ireland.
Liberal Democrat leader Tim Farron labelled the Tory-DUP arrangement as a "shoddy little deal", adding: "While our schools are crumbling and our NHS is in crisis, Theresa May chooses to throw cash at 10 MPs in a grubby attempt to keep her Cabinet squatting in Number 10."
And the SNP's leader in Westminster, Ian Blackford, said the deal showed Scotland is "seemingly to be offered little more than scraps from the table".
However, Scottish Conservative leader Ruth Davidson has defended Mrs May - and said the UK Government has always been able to spend outside the Barnett formula, which is the mechanism used to distribute Treasury funds to the devolved nations.
She explained that £500m was invested in Glasgow outside of Barnett by politicians in Westminster, adding: "It's absurd for the SNP to criticise UK Government spending on top of Barnett in Northern Ireland when the exact same thing happens in Scotland."
