US House Speaker Paul Ryan told Republicans in the House of Representatives on Wednesday that he will not seek re-election in November, his office said.
Mr Ryan will serve his full term and retire in January, Brendan Buck, spokesperson for the Speaker, said in a statement.
"After nearly twenty years in the House, the speaker is proud of all that has been accomplished and is ready to devote more of his time to being a husband and a father," Mr Buck said.
The departure of 48-year-old Mr Ryan could complicate Republican Party efforts to retain the House in November, when candidates may be dragged down by the unpopularity of President Donald Trump.
The announcement of his departure months before the election will give potential candidates for House Republican leadership positions plenty of time to campaign for support.
The House speaker has scheduled a news conference for 10 am local time.
Reports of Mr Ryan's departure have circulated for months. Politico reported in December that he had told confidants he would like to retire after the 2018 congressional elections.
Friends said Mr Ryan, a longtime champion of tax reform, was ready to step down after passing a tax reform bill, according to the Axios news site, which first reported on Wednesday that Mr Ryan would soon announce his retirement.
The tax bill was Mr Trump’s first major legislative victory since he took office in January 2017 despite being helped by Republican control of Congress.
Lawmakers had expected Mr Ryan might leave Congress if Republicans lose the House in November. The early announcement could have an impact on Mr Ryan's ability to raise campaign funds for Republican candidates.
More than three dozen House Republicans have said they are retiring, or running for another office, or resigning. Democrats need to win 23 seats in the November elections to retake a majority in the House, which Republicans have controlled since 2011.
Democrats believe that voter concerns over rising medical costs and Republican plans to cut Medicare and Medicaid will assist them in their fight to retake the House.
President Trump said in a post on Twitter: "Speaker Paul Ryan is a truly good man, and while he will not be seeking re-election, he will leave a legacy of achievement that nobody can question. We are with you Paul!"
Mr Ryan was elected to the US House of Representatives in 1998 from Wisconsin at age 28 and was quickly pegged as a Republican rising star. He became 2012 Republican presidential candidate Mitt Romney's vice presidential running mate, but Mr Romney was beaten by incumbent Democratic President Barack Obama.
In Congress, Mr Ryan earned a reputation as a fiscal policy expert, serving as chairman of the House Budget Committee from 2011 until 2015, but as speaker was a driving force behind a Republican tax overhaul passed by Congress last year that is projected to balloon the federal deficit.