Long‐term changes in historical observations and future projections of reference evapotranspiration (ET₀) are of great importance in assessing the potential impacts of climate change on the hydrologic regime as well as water resources systems. In this study, with the aid of nonparametric trend analysis and geographic information system methods, spatial and seasonal patterns of changes in ET₀ across the Tibetan Plateau (TP) were investigated using meteorological data of 81 stations for the period of 1961–2010. Almost half of all stations in the plateau, most of which were distributed in the north plateau, and the southwest and southeast corners, were characterized by decreasing trends in ET₀ during 1961–2010 at both annual and seasonal scales. The temporal regional average series presented a zigzag increasing–decreasing–increasing pattern with two joint points in 1973 and 1993. The increasing vapor pressure deficit and air temperature dominated the increase in ET₀ in the past two decades. The intrinsic mechanism term of hydrological feedback was attempted to be explored by examining the applicability of the Bouchet's hypothesis and Budyko‐type expressions in the TP. Complementary effect existed between potential evaporation and actual evapotranspiration, deviating from theoretic relationship of Bouchet's hypothesis. Water‐limited and energy‐limited evaporation patterns at mean annual scale can be described well by Budyko‐type expressions in the TP, especially Zhang's curve. Changing characteristics of the projected ET₀ on the plateau during 2011–2100 from the HadCM3 (Hadley Centre Coupled Model version 3) under A2 and B2 emission scenarios and CGCM3 (the third generation couple global climate model) under A2 and A1B emission scenarios were projected based on a statistical downscaling method (SDSM). The SDSM performed fairly well in reproducing ET₀. The continuous increase in ET₀ in the 21st century was revealed by both climate models. Concurrently, a larger increasing magnitude in ET₀ was generally projected by HadCM3 compared with that by CGCM3. Spatially, projected annual ET₀ changes under the different scenarios are similar with the increments in the north and southeast corners of the plateau and low increments over the southwest, northwest, and middle of the plateau. The results can provide beneficial reference to agriculture, water resource, and eco‐environment management strategies in the plateau region for associated policymakers and stakeholders.