Owen Smith congratulated Jeremy Corbyn on "being elected decisively as our leader". Union leaders also welcomed Corbyn's re-election, but opposition parties were quick to pour scorn on the result.
Smith said he was "proud and privileged to have stood to lead our party and to have won the support of 193k members and supporters."
He added: "Now is time for all of us to work to take Labour back to power."
There was widespread endorsement of the result among British unions, many of whom had backed Corbyn for a second time.
Dave Prentis, general secretary of Unison, said he "won because he's captured the imagination of party members", while Mick Whelan, leader of the train drivers' union Aslef, added that it was time for the party to get behind him.
Calling the leadership contest "unhelpful", Prentis said people were inspired by Corbyn's "promises to end austerity, fix our broken public services and build a different kind of economy".
But he warned that the "scale of the political challenge facing Labour cannot be ignored".
Whelan, meanwhile, said it was "time for everyone – especially those in the Parliamentary Labour Party (PLP) who have spent so much of the last year undermining Jeremy – to get behind him, to turn their fire on the Tories, and to get ready for a general election which could be just around the corner".
His views were echoed by one of Corbyn's key backers, the leader of Unite, Len McCluskey, said Labour MPs "should now listen to its members and stop infighting.
Calling the election a needless distraction he said the union would urge them "to heed the signal sent by the members – twice now in one year – about the direction they want for the party.
"This includes respecting and supporting the elected leader and his team; no more sniping, plotting and corridor coups."
Even the GMB union, which had supported Smith, offered their congratulations to Corbyn, saying it was time to "stop banging on about our internal differences".
Carolyn Fairbairn, director-general of the Confederation of British Industry, said they looked forward to continuing to work with Corbyn and his team.
"It's critical that the country has a strong opposition at a pivotal moment in our nation's history," she said.
Lib Dems and Tories attack
However, the news was not universally welcomed.
Liberal Democrat leader Tim Farron said it was "a case on things can only get worse".
He said Corbyn had "failed as an opposition leader and failed to stand up for Britain's place in Europe. He is now not backing our membership of the single market despite the damage leaving would do to our economy and the threat it poses to jobs."
Conservative Party chairman Sir Patrick McLoughlin also dismissed the news.
"Labour are too divided, distracted and incompetent to build a country that works for everyone," he said, adding that 172 "Labour MPs don't think Jeremy Corbyn can lead the Labour Party - so how can he lead the country?"
However, Communist Party general secretary Robert Griffiths said, "Corbyn's victory will be welcomed by socialists and progressives throughout the labour movement."
Sinn Fein president Gerry Adams also extended his "warmest congratulations to Jeremy on his resounding win."
Calling Corbyn a "stalwart friend to Ireland and of the Irish peace process for decades," he warned that he "faces many challenges in the time ahead in confronting the right wing agenda of the Conservative government, not least in addressing the issue of Brexit."
However, Scottish Labour leader Kezia Dugdale warned Corbyn he faces a "difficult task" to reunite the party in wake of a divisive leadership election.
Dugdale, who backed Owen Smith in the leadership election, said Corbyn needed "to work with both the party across the country and MPs to provide an effective opposition to the Tories in Westminster."
But Scottish Conservative leader Ruth Davidson said his election would be "greeted with utter dismay by thousands of moderate voters across Scotland and the rest of the UK", while Scottish National Party (SNP) business convener Derek Mackay claimed Labour was "completely and irreparably divided from top to bottom - and this division is the greatest gift the Tories could ever ask for."