Reference evapotranspiration is a key parameter in hydrological and meteorological studies and used to determine the actual water use rate for various crops. The objectives of this study were to explore trend in the grass-reference evapotranspiration (ETo) through years 1961–2011 and to identify trend in the aridity index as an indicator of change in climate in Togo. ETo was calculated using the FAO-56 Penman–Monteith method, and trends analyses were performed with non-parametric statistics proposed by Mann–Kendall and the Sen slope estimator. Results showed that annual ETo varied from 1,440 to 1,690 mm at Lomé, from 1,761 to 1,905 mm at Tabligbo, and from 1,839 to 1,990 mm at Sokode. The Mann–Kendall test revealed significant increase in annual ETo at Tabligbo (Z = 2.89) and Sokode (Z = 2.29). Annual ETo is much more stable at Lomé, with non-significant decrease. In Togo, according to the three study sites, the 1961–2011 period annual aridity index varied from 0.26 to 0.99 at Lomé, 0.38 to 0.98 at Tabligbo, and 0.45 to 1.08 at Sokode. The Mann–Kendall test revealed a declining trend in the ratio of precipitation/ETo which adversely implies an increasing severity of the aridity index at all the sites, prejudicial to rainfed agriculture practiced by about 90% of Togolese crop growers.