This study investigated the water hammer pressure due to the sudden closure of the partially open valve experimentally and analytically. Because the partially open valve could produce local nonuniform flow, a supplementary Joukowsky's water hammer equation was derived based on the assumption of the local nonuniform flow and the kinetic energy equation. A physical model was set up to measure the maximum water hammer pressure of the first positive wave due to the sudden closure of the partially open valve under different conditions, including various water heads, flow velocities, pipe diameters, and valve types. The results showed that Joukowsky's equation obtained by the momentum theorem in the uniform flow field was applicable to the uniform flow field with the valve fully open. The experimental results of the partially open valve-closure water hammer pressures were 3.5–21% larger than Joukowsky's equation, which consisted of the theoretical analysis of the supplementary Joukowsky's water hammer equation. This phenomenon had repeatability and was unrelated to the water head, the inlet flow velocity, the pipe diameter, and the valve type. This study could guide water hammer protection in hydropower and pump stations.