An ammonia synthesis catalyst synthesizing ammonia from nitrogen in a presence of moisture is provided. The ammonia synthesis catalysis includes a catalyst particle including an inorganic material that has a photocatalytic function and an inorganic acid. The catalyst particle is preferably an n-type semiconductor and includes oxide material including at least titanium preferably. The inorganic acid preferably corresponds to at least one of perchloric acid, hydrochloric acid, sulfuric acid, and phosphoric acid.