The decision comes days after voters flocked to Democratic candidates in state elections in Kentucky and Virginia, underscoring Republican vulnerability in a suburban revolt against President Donald Trump.
But in the latest indication that congressional Republicans are rallying behind Trump for now, King said he will oppose Democratic efforts to impeach Trump. King also opposed Bill Clinton's impeachment in 1998.
"In the coming weeks and during the next year I intend to vote against President Trump's impeachment and will support the President's bid for re-election," King said.
Read his full announcement:
The 14-term congressman, 75, said in a Facebook post that he's retiring because he wants "flexibility to spend more time" with his children and grandchildren "after 28 years of spending 4 days a week in Washington, D.C."
King, a former chair of the House Homeland Security Committee, has cultivated a reputation for bipartisanship while maintaining a hard line on immigration and crime. He is the longest-serving Republican member of New York's congressional delegation. Still, he won reelection in 2018 by just 6 percentage points.
King's keen political instincts and reputation as a fighter for New York's interests gave him remarkable political resiliency; at one point, after the 2008 election that sent Barack Obama to the White House, he was the state's lone GOP member of Congress.
He teamed up with powerful Democrats such as Chuck Schumer to win a huge Superstorm Sandy aid package after the 2012 storm, despite foot-dragging by GOP leaders. On Monday, the Senate Democratic leader took to Twitter to lavish praise on King.
"Peter King stood head & shoulders above everyone else," Schumer wrote. "He's fiercely loved America, Long Island, and his Irish heritage and left a lasting mark on all 3."
His district includes once-reliably GOP territory in southwestern Suffolk County and a portion of Nassau County, about an hour's drive east of Manhattan. It went narrowly for Trump in 2016.
Rep. Cheri Bustos, D-Ill., who heads House Democrats' election arm, said King's retirement "underlines just how serious Republicans' problems are in swing districts across this country." While her party has long targeted his district, she added, "We will compete to win it in 2020."
Many suburban districts around the country have been moving steadily toward Democrats as moderate, well-educated voters swing away from the polarizing president.
Twenty House Republicans have announced they will not seek reelection. Three other GOP lawmakers have resigned and already left Congress.
Only a handful of Republican-held districts being vacated by retirements are expected to be seriously competitive next year, but King's will certainly be among them.
House Democrats retook the majority in 2018, aided by victories in what had been GOP-held suburban seats from Virginia to California.
New York State Republican Party Chairman Nick Langworthy praised King and said, "We have every confidence we will retain this seat and continue the strong representation that Long Islanders deserve."
* More Long Island news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube