Peter Hain has been given a promotion by new Prime Minister Gordon Brown as he unveiled a raft of changes in his first cabinet.
Peter Hain is said to be delighted by the promotion
Downing Street confirmed that the Neath MP will become work and pensions secretary, replacing John Hutton.
Mr Hain said he was also "delighted" to be continuing as Welsh secretary.
Mr Hain, who came fifth last weekend in the deputy Labour leadership contest, moves from the job as Northern Ireland secretary.
There had been speculation that Mr Hain could take over from Hilary Benn as international development secretary but that post has gone to Douglas Alexander, former transport and Scottish secretary.
The Department for Work and Pensions is one of the top-spending departments in Whitehall.
The department - once known as the department for social security - is responsible for all sorts of benefits, from allowances paid to carers to child support, incapacity benefit and, of course, pensions.
Welfare reform was one of the big issues Tony Blair highlighted on coming to power 10 years ago.
His first welfare reform minister was told to "think the unthinkable" - and sacked a year later.
Among the challenges facing Mr Hain will be the government's drive to reduce the number of people on incapacity benefit - a problem common in former industrial areas of Wales.
He will also come under fresh pressure to improve financial assistance to the former workers at ASW in Cardiff and elsewhere who lost their pensions when their company went bust.
But one of the department's aims is to reduce poverty. Mr Hain has regularly called for the gap between rich and poor to be narrowed.
The MP said he was "delighted" after a week in which his Northern Ireland job was offered to Paddy Ashdown and The Sun predicted he would be sacked.
Following the announcement, he said: "It is a great honour to be asked by Prime Minister Gordon Brown to continue to represent Wales in the Cabinet at such a crucial time for devolution in Wales.
"We are entering an exciting new era for devolution in Wales. I look forward to continuing to oversee the smooth implementation of the additional powers to the Assembly through the Government of Wales Act.
"As secretary of state for Wales, I believe we must embrace the opportunities presented by these new law-making powers to address the challenges of our changing world.
"It is essential that the government in Cardiff Bay continues to work in partnership with the government in Westminster on key UK national priorities, such as the economy and climate change."