He gave the pledge as he returned to Downing Street after the Conservatives won the general election.
The prime minister said he would continue to govern with "respect" towards the administrations in Wales, Scotland and Northern Ireland.
But the Lib Dems claimed they had delivered further powers against "much resistance" from the Tories.
Mr Cameron said "the governance of these nations will become powerful with wider responsibilities".
The prime minister announced in February plans to give the Welsh assembly more powers over energy projects, its own elections and scope for borrowing on the financial markets.
Following his party's victory, Conservative Welsh Secretary Stephen Crabb told BBC Wales: "I think devolution and the relationship between the different nations of the United Kingdom will be a big theme in the next parliament, in the coming five years.
"Now we know in Wales there's a new piece of legislation already being written by my officials in the Wales office. That's going to strengthen the devolution settlement in Wales.
"It's also going to make devolution fairer for Wales because we are absolutely committed to following through on this historic commitment to bring fair funding to Wales; to make devolution work better in the interests of Welsh people and the Welsh economy."
However, A Welsh Lib Dem spokesperson said: "We've spent the last five years delivering further powers for Wales - often against much resistance within the Tory party.
"David Cameron must deliver all of the St David's Day agreement and that includes fair funding with no strings attached.
"The Tories are all over the place on this issue and it is time they clarified their position."
Labour and Plaid Cymru have also been asked to comment.
David Cameron has promised to press ahead with plans to devolve more power to Wales "as fast as I can".