The aim of this paper is the experimental characterization of a ball valve under steady and unsteady conditions, based on experimental tests carried out in a pressurized pipe system assembled at Instituto Superior Técnico. Data analysis showed that the response of the valve under steady-state conditions depends not only on the valve geometry and closure percentage, but also on the flow regime (i.e., Reynolds number); the comparison with values published in literature confirmed that typical valve head loss curves presented in manuals and textbooks refer to turbulent flows. The behavior under unsteady conditions was also analyzed based on transient pressure head measurements showing that the valve effective closure time varies between 4 and 10% of the total time of the maneuver. Two mathematical functions (hyperbolic and sigmoidal) are proposed to describe the discharge variation as a function of the total time of the maneuver and of the initial steady-state discharge; the most appropriate function depends on the simplifications imposed on the hydraulic transient simulator. A computational fluid dynamics (CFD) model was used to support the observed experimental behavior and to highlight the effect of the pipe length on the discharge variation.@en