New Labour leader Sir Keir Starmer "will stand up for Wales", First Minister Mark Drakeford has said.
Sir Keir won the race to succeed Jeremy Corbyn with 56.2% of the vote, beating Rebecca Long-Bailey and Lisa Nandy.
Mr Drakeford, who is Welsh Labour Party leader, said Sir Keir's leadership would be "crucial" in the coming months.
He also called on the party to unite to face the "challenges ahead".
"Keir becomes leader at a critical time for our country," Mr Drakeford said.
"His leadership in Parliament will be crucial in the coming months as we respond to the coronavirus outbreak, and then as we seek to build the more equal and just society that must surely follow.
"I know that in Keir, we have a UK Labour leader who will continue to stand up for Wales' interests and support the work of the Welsh Labour government in delivering for the people of Wales."
Labour lost six Welsh seats to the Conservatives in the December general election, including some in traditional heartlands such as Bridgend and Wrexham.
Mr Drakeford said the party must now unite behind the new leader and gain the public's trust.
"United and focused we will win the public's trust and, in time, their permission to govern across the UK."
During a leadership hustings in Cardiff in February, Sir Keir said more powers should be devolved to Wales.
He added that he wanted to see the Welsh leadership play a "bigger part in decision making" in the party.
Swansea East MP and Welsh Labour deputy leader Carolyn Harris said: "When we overcome the challenges of today, and overcome them we will, Labour will stand united, ready under Keir's leadership to win back the trust of communities and return the Labour government we so desperately need."
Shadow Welsh Secretary Christina Rees MP said Wales had a "firm supporter in Keir".
"I know from working alongside him in the shadow cabinet that he gets devolution, works closely with our Welsh Labour government, and understands the importance of always respecting both," she said.
"Keir's immediate focus, like all of ours, is on tackling the Covid-19 pandemic, and he will do this with both compassion and with his trademark forensic attention to detail."
Analysis by Cemlyn Davies, BBC Wales political correspondent
Keir Starmer accepts Labour has "a mountain to climb" as he takes over as leader - the party suffered its worst general election defeat since 1935 in December, losing six seats in Wales.
For now his focus must be on the Covid-19 crisis, during which he has promised to "engage constructively" with the Conservative UK government.
Next year's devolved elections in Wales and Scotland will be key tests of his impact, with opinion polls currently suggesting Labour will lose seats in Cardiff Bay.
At least he begins his leadership on a sound footing, enjoying widespread support amongst his own MPs.
Thirteen Welsh Labour MPs nominated Sir Keir. So did former MEP Jackie Jones.
For comparison, just one Welsh Labour MP nominated Jeremy Corbyn in 2015, while another challenged Mr Corbyn in a leadership contest in 2016.