The party overtook the Democratic Unionist Party (DUP), whose losses included Ian Paisley Jr, the son of former Northern Ireland First Minister and party founder Ian Paisley.
The father and son had held a seat at Westminster since 1970.
The victory marked an electoral clean sweep for Sinn Fein, which in 2022 became the first nationalist party to win the most seats at the regional assembly.
It won at local council polls a year later.
"There's no doubt the landscape is changing on our island," Sinn Fein's Michelle O'Neill, Northern Ireland's first minister, told the BBC on Friday.
The former political wing of the Irish Republican Army (IRA) retained seven of Northern Ireland's 18 seats Westminster, despite the party's long-standing policy of not attending parliament.
The DUP, which fought the election just three months after the shock resignation of leader Jeffrey Donaldson, charged over historical sex offences, fell to five seats from eight, its worst result since 2001.
The party was under pressure from hardline unionists over a deal it struck with the British government to ease post-Brexit trade rules.
The deal enabled February's restoration of Northern Ireland's devolved assembly after a two-year unionist boycott.
Paisley lost his seat to Jim Allister, the firebrand leader of the much smaller Traditional Unionist Voice (TUV), who called the win "a political earthquake."
He accused the DUP of trying to "hoodwink" voters into believing it had ended checks on goods between Northern Ireland and the rest of the United Kingdom.
Many unionists see the trade frictions as a threat to their British identity.
The DUP also lost a seat to the more moderate Ulster Unionist Party and one to the cross-community Alliance Party.
For Sinn Fein, the victory also offered some relief after its ambition of governing on both sides of the Irish border suffered a setback when it saw a commanding three-year opinion poll lead in the Republic of Ireland collapse at local council elections. Parliamentary elections are due within months.
O'Neill said the massive majority won by Keir Starmer's Labour Party in Britain would be an opportunity to reset London's relations with both Belfast and Dublin, a point echoed by the Irish government.
Foreign Minister Micheal Martin said successive Conservative Party governments "didn't really embrace" the spirit of the 1998 Good Friday Agreement that ended 30 years of sectarian violence in Northern Ireland.
"It is time for a great reset," Prime Minister Simon Harris said.
"This morning from Dublin, I want to send a message to London that I will match Keir Starmer's commitment and energy to our peace process and to our future potential in so many areas."
