The Flyers also received a fifth-round pick in the 2024 NHL Draft.
Lemieux has three points, all assists, in 27 games this season. The forward signed a one-year, $1.35 million contract with Los Angeles on July 13, 2022, and can become an unrestricted free agent after the season.
RELATED: [2022-23 NHL Trade Tracker]
The 26-year-old has 63 points (31 goals, 32 assists) in 257 NHL games with the Kings, New York Rangers and Winnipeg Jets. He was selected by the Buffalo Sabres in the second round (No. 31) of the 2014 NHL Draft.
"Brendan is a pending UFA and we can give him a 20-game trial to see what he can do," Flyers general manager Chuck Fletcher said.
Fletcher said the trade was about acquiring more draft capitals for the Flyers.
"The deal was for the fifth," Fletcher said. "[Kings GM] Rob [Blake] had indicated that for whatever his reason if he'd move Brendan he'd do the deal. I thought I'd make it easy for him."
MacEwen has nine points (four goals, five assists) in 46 games this season, which matches his previous NHL best of nine points (three goals, six assists) in 75 games with the Flyers last season, who claimed the forward on waivers from the Vancouver Canucks on Oct. 13, 2021.
MacEwen has been out because of a broken jaw sustained Jan. 26 but was activated from injured reserve Friday.
The 26-year-old signed a one-year, $925,000 contract Aug. 2, 2022, and can become a restricted free agent after the season.
MacEwen has 27 points (13 goals, 14 assists) in 176 NHL games with the Flyers and Canucks.
The Kings (35-20-8) have won two straight games and seven of their past 10 (7-2-1) to move into a tie with the Vegas Golden Knights for first place in the Pacific Division. They previously acquired defenseman Vladislav Gavrikov and goalie Joonas Korpisalo in a trade with the Columbus Blue Jackets for goalie Jonathan Quick on Wednesday.
The Flyers (23-28-11) have lost eight of their past nine games (1-6-2) and are seventh in the Metropolitan Division.
Philadelphia also traded forward Patrick Brown to the Ottawa Senators for a sixth-round pick in the 2023 NHL Draft. Brown has seven points two goals, five assists) and has won 53.8 percent of his face-offs in 43 games this season.
NHL.com deputy managing editor Adam Kimelman contributed to this report.
