•Self-supported Pt-Ni(OH)2-Ni foam electrode was fabricated via hydrothermal synthesis method.•The electron sturcure and high-quality ohmic contact between Pt and Ni(OH)2 were investiged.•A novel ammonia–nitrogen sensor was fabricated based on Pt-Ni(OH)2-Ni foam electrode.•This sensor showed great detection performances. In this work, Ni foam-supported Pt-Ni(OH)2 nanosheets (Pt-Ni(OH)2-Ni foam) was designed to construct a novel electrochemical sensor for detection of ammonia–nitrogen in water. Pt-Ni(OH)2-Ni foam was prepared via one step and in situ hydrothermal synthesis in chloroplatinic acid solution and Ni foam acted as Ni source and conductive substrate. The electrochemical measured results indicated that Pt-Ni(OH)2-Ni foam showed excellent catalytic activity to electrooxidation of ammonia due to great synergistic effect between Pt and Ni(OH)2. Furthermore, the constructed electrochemical sensor based on Pt-Ni(OH)2-Ni foam exhibited great detection ability to ammonia–nitrogen with the linear range from 0.05 μM to 600 μM. The limit of detection was 39.2 nM (Sensitivity/Noise = 3). The optimized electron sturcure and high-quality ohmic contact between Pt and Ni(OH)2 were main reasons of great performances of Pt-Ni(OH)2-Ni foam. This work provided a potential sensor for ammonia–nitrogen determination.