Arsenal lost at home in the Premier League for the first time this season as West Ham United dealt a major blow to their title aspirations.
The Gunners went into Saturday's game eight points behind league leaders Liverpool, who had played one match more.
Mikel Arteta's men now remain eight points behind, with just 12 rounds of fixtures remaining.
Jarrod Bowen fired the Hammers in front just before half-time by scoring his eighth goal of the campaign in all competitions.
Arsenal dominated possession as they tried to battle their way back into the contest but that task got harder in he 73rd minute when Myles Lewis-Skelly was shown a straight red card for a foul on Mohammed Kudus that denied the West Ham ace a clear scoring chance.