The company said Mr. Kleinfeld's decision to step down had nothing to do with the proxy fight being waged by Elliott, a New York-based hedge fund. Arconic's statement praised Mr. Kleinfeld for his "transformative vision" and said he improved the company's performance despite a hard environment. The letter "showed poor judgement", Arconic said in a statement.
Elliot Management holds a 13.2% stake in the lightweight metals manufacturer.
Arconic's stock jumped as much as 9.8 percent on Monday, before easing to trade up 2.5 percent at $26.56 in late afternoon trading. She was lead director on the Arconic board since it separated from Alcoa Inc.
Klaus Kleinfeld has been pushed out as chairman and chief executive officer of Arconic Inc., an abrupt departure following heavy pressure from activist investor Elliott Management Corp.
Arconic, which split off from Alcoa Inc. last year, operates Arconic Davenport Works in Riverdale, the 10th largest employer in the Quad-Cities, according to Quad-City Chamber of Commerce data.
Elliott reported in March that Spirit AeroSystems had said Lawson breached a non-compete clause in his retirement contract by consulting for Elliott, a view the hedge fund has said it disagreed with.
That facility was part of Alcoa before the two companies separated.
Elliott was not immediately available for comment.
The company added that since Elliott's central objective - a CEO change - had been realized, the hedge fund should not "continue to burden Arconic and its shareholders with its highly disruptive and distracting proxy fight". Alcoa Corp's shares had risen 34.7 percent.