The Rockets' clash against the Oklahoma City Thunder, scheduled for Wednesday, was postponed after three Houston players returned tests that were either positive or inconclusive for coronavirus.
Harden, who has reportedly requested a trade, was also unavailable after attending a private indoor party on Monday, a violation of the NBA protocols.
The NBA confirmed on Wednesday that the eight-time All-Star had been fined, but avoided a suspension.
"Houston Rockets guard James Harden has been fined $50,000 for violating the league's health and safety protocols, which among other things prohibit attending indoor social gatherings of 15 or more people or entering bars, lounges, clubs or similar establishments, it was announced today by Byron Spruell, president, league operations," a statement read.
"Harden violated these rules when he attended a private indoor party on Monday, December 21."