Plant fossils from the Qinghai-Tibetan Plateau (QTP), China are critical to understand not only the diversification history of plants there, but also the paleoenvironmental conditions. Alnus are deciduous trees, mainly distributed in temperate and subtropical regions of Eurasia and North America, and they are well known in the fossil records throughout the Cenozoic in the Northern Hemisphere. We collected numerous well-preserved Alnus leaf and infructescence fossils from the Lawula Formation (similar to 34.6 Ma with Ar-40/Ar-39 dating) at the present elevation of 3910 m a.s.l. in the southeastern QTP. Based on detailed morphological comparisons with existing and fossil species, these fossils show closest affinity to Alnus ferdinandi-coburgii C. K. Schneid., and we refer to these fossils as A. cf. ferdinandi-coburgii. These specimens comprise the oldest megafossil record of Alnus in the QTP, and provide solid evidence for the distribution of Alnus there as early as the late Eocene. Extant A. ferdinandi-coburgii is distributed in areas with mean annual temperature values between 9.7 degrees C and 16.9 degrees C, and mean annual precipitation values ranging from 896.2 mm to 1161.2 mm; therefore, fossils of A. cf. ferdinandi-coburgii suggest a much warmer and wetter climate during the late Eocene than today in the southeastern QTP. This finding is consistent with other evidence for continued uplift of the southeastern QTP after the late Eocene that might be due to the eastward extension of the QTP.