In a statement Monday, Arconic says Kleinfeld "showed poor judgment".
The ouster comes after the company's board learned that he sent a letter directly to Elliott Management, an institutional investment group that has been pushing for Kleinfeld's ouster, without authorization from the board.
Elliot Management holds a 13.2% stake in the lightweight metals manufacturer.
Arconic's stock price has jumped on the news, rising about $1 in value from $26 per share at the start of Monday morning, according to Markets Insider data.
April 17 US specialty metals maker Arconic Inc's chief executive, Klaus Kleinfeld, resigned on Monday amid attempts by activist hedge fund Elliott Management to replace the company's directors and top executives.
Arconic also said its board reaffirmed the strategy developed by Kleinfeld, under whose leadership, it said, the company improved its performance.More news: Rose, Garcia tied for lead at Masters in prelude to finale
David Hess, a former United Technologies Corp. executive who was named to Arconic's board last month, was named interim CEO of Arconic.
In 2015, shortly before Alcoa announced their plans to close their Massena smelter, the company announced plans to split into two corporations - Alcoa and Arconic. The New York-based company was spun off from aluminum company Alcoa past year.
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".
The hedge fund reported in a regulatory filing in March that Spirit had said Lawson breached his retirement contract by consulting for Elliott.
Elliott was not immediately available for comment.