This paper reports comprehensive and accurate measurements of the speed of sound in pure n-butane in the liquid region. The measurements were carried out by a double-path-length pulse-echo technique and cover the temperature range between 200 and 430 K with pressures up to 100 MPa. The expanded uncertainties (at the 95% confidence level) are 2.1 mK for temperature, 0.007% for pressure, and 0.016% for speed of sound. Comparisons with several equations of state and literature data demonstrate that the Helmholtz energy formulation for n-butane can be improved significantly with our accurate data.