Adam Price is widely regarded as a likely future leader of Plaid Cymru
A Plaid Cymru MP often tipped as the next party leader will leave Parliament at the next election, before he hopes to join the Welsh assembly.
Adam Price, who represents Carmarthen East and Dinefwr, will first take up a year-long Fulbright scholarship in the United States.
He then wants to stand in the 2011 Welsh assembly election.
Mr Price, aged 40, said "the time is ripe for change" after two terms at Westminster.
In a statement, Mr Price said he was "incredibly honoured" to represent his seat.
But he added: "I do not think it would be right to return to Westminster at the next election when my passion is in Wales and at the assembly.
"My aim will be to seek nomination and stand for election in 2011 as part of [party leader] Ieuan Wyn Jones's assembly team.
He said he was looking to spending the year after the general election "studying in America as part of the Fulbright programme in order return to politics in Wales with greater experience and innovative radical ideas".
Ieuan Wyn Jones, the leader of Plaid, said the MP had been "outstanding" in Westminster and the US scholarship was "further recognition of Adam's talents and his belief that we in Wales should take every opportunity to learn and share ideas and best practice from across the globe".
He added that Mr Price had his "full support" in achieving his aim of being elected to the assembly.
The Fulbright Commission website says it "offers a selection of awards for UK citizens looking to study in the US and for US citizens coming to the UK".
Mr Price, widely regarded as a future leader of Plaid Cymru, was one of the authors of the One Wales Agreement, the basis of the Labour-Plaid coalition Welsh Assembly Government formed two years ago.
But in March he embarrassed Plaid's leadership when he alleged ministers plans to scrap a grant covering the cost of student "top-up" fees breached the spirit of the coalition deal and could be illegal.
In 2004 Mr Price led a high-profile attempt to impeach former Prime Minister Tony Blair over the Iraq war.
He won the Carmarthen East and Dinefwr seat in the 2001 general election, Plaid's only gain in that year, and increased his majority in 2005.