Royce's Orange County district is now considered vulnerable in the midterm elections.
Royce, who has served in the House for almost 26 years, was expected to face a tough re-election battle if he had chosen to run again.
Royce, chairman of the House Foreign Affairs Committee, did not say explicitly why he's not running.
Royce won in 2016 with more than 57 percent of the vote, and Republicans hold a narrow edge in voter registration.
Royce, 66, who represents parts of Los Angeles, Orange and San Bernardino Counties, said he plans to focus on "urgent threats facing our nation" - including "brutal, corrupt and risky regimes" in North Korea and Iran. However, California has been growing increasingly favorable to Democratic candidates.
The 39th district consists of the most northern portion of Orange County from parts of Buena Park, Placentia and Anaheim Hills and contains all of Yorba Linda, Brea, La Habra and Fullerton while reaching north to portions of neighboring Los Angeles and San Bernardino counties. In 2012, Republicans enjoyed almost eight percentage points over Democrats in the district.
"In the suburbs, Trump is toxic", Pitney said.
Eddie Kurtz, executive director of the left-leaning Courage Campaign, also painted Royce's announcement as a reflection of the political danger ahead for California Republicans. In order to take control the House, Democrats need to gain 24 seats in November.
Royce touted his accomplishments in a statement.
Republican Rep. Steve Stivers, the chairman of the National Republican Congressional Committee, said the GOP was "fired up and ready to hold this seat".
Incumbents who don't think that a wave is coming against them don't retire in such high numbers.
If Republicans maintain control of the House in the November elections, Representative Michael McCaul, a foreign affairs committee member who is now chairman of the House Homeland Security Committee, is seen as a possible new leader of the foreign affairs panel, congressional aides said.
"In a district that Hillary Clinton won by 8 points, Democrats are poised to win this Orange County seat and send someone to Washington who truly represents the values of Southern California", said Drew Godinich, a spokesman for the Democratic Congressional Campaign Committee. The president's party usually experiences losses in the mid-term election.
And Harper's announcement followed retirement plans announced by Senate Finance Committee Chairman Orrin Hatch, R-Utah, and House Transportation and Infrastructure Chairman Bill Shuster, R-Pa. Simple: He's term-limited as committee chair.