Global circulation models predict an increase of summer monsoon precipitation in High Asia as a consequence of global warming. The shortness of available meteorological records requires the reconstruction of past climate variability. However, high-resolution climate proxy records from the Tibetan plateau are scarce and of limited spatial representativeness. Here we present first evidence of increased summer monsoon intensity from the Tibetan plateau based on reconstructions of late summer (August and September) temperature and rainfall from a network of 22 maximum latewood density (MLD) chronologies of high-elevation conifer sites. After 1980, a decrease in MLD points to an increase of Indian summer monsoon activity in southern Tibet unprecedented during the past 350 years.