The speed of sound in liquid cyclopentane has been measured in the temperature range from (258 to 353) K at pressures up to 30 MPa (42 data points) using a pulse-echo method with a double path type sensor. The expanded overall uncertainty (k = 2) in the speed of sound measurements is estimated to be 0.2 %. A function for the speed of sound with inputs of temperature and pressure has been fitted to the experimental results. The new speed of sound data along with available literature data were used to develop a fundamental Helmholtz equation of state for cyclopentane. Typical expanded uncertainties of properties calculated using the new equations are 0.2 % in density in the liquid phase, 1 % in heat capacity, 0.2 % in liquid-phase sound speed, and 0.5 % in vapor pressure. The equation of state is valid from the triple-point temperature, 179.7 K, to temperatures of 550 K with pressures to 250 MPa.