The three parties will announce details of the deal on Thursday, but it is thought to cover between 60 and 70 constituencies, reports the BBC.
Such a pact means two of the parties would agree not to field a candidate, boosting the third candidate's chances.
In Wales, the pact will cover 11 of its 40 seats. Outside of Wales, the pact will simply be a two-way agreement between the Liberal Democrats and the Greens.
All three parties have already launched their election campaigns. They support another Brexit referendum and want to Remain in the European Union (EU).
Speaking after the formation of the pact on Wednesday, Liberal Democrats leader Jo Swinson said: "We are delighted that an agreement has been reached... This is a significant moment for all people who want to support Remain candidates across the country."
The pact comes after Plaid Cymru's leader Adam Price wrote to several pro-Remain parties earlier this year, calling on them to work together in a snap general election.
Last week, the Brexit Party's Nigel Farage called on Prime Minister Boris Johnson to form a similar election pact, the BBC said.
But Johnson rejected the offer and said he would not enter into election pacts.
Labour leader Jeremy Corbyn has also ruled out the idea of entering electoral pacts with rival parties.
( With inputs from IANS )