A global comparison of altimeter and ERA-40 mean wave period data is performed for the first time. Altimeter and ERA-40 mean wave period are shown to compare well in non-swell-dominated conditions. Triply collocated altimeter, buoy, and ERA-40 data show that the variance of the random errors is bigger for the ERA-40 data and smaller for the buoy data. A new expression is given to relate altimeter data to wave period in wind-dominated conditions. Provided this correction is applied, altimeter data are adequate for the most needed validation of global model mean wave period data in non-swell-dominated conditions. Triply collocated altimeter, buoy, and ERA-40 also reveal that altimeter mean wave period is reliable even in swell conditions for moderate to high winds, in contrast to previous suggestions which limited its validity to wind-dominated seas.