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The decreasing trend in diurnal temperature range (DTR) since the 1950s has been widely observed in many regions of the world, including the Tibetan Plateau (TP). However, scarce instrumental records on the TP before the 1950s limit the understanding of DTR variation on a longer timescale, and its internal forcing mechanisms remain unclear. Here, we present two regional records of May–June DTR since 1753 reconstructed from tree rings on the northeastern TP (NETP) and southeastern TP (SETP), respectively. The decreasing trend in DTR in the second half of the twentieth century also occurred in the time earlier than the 1950s, indicating that the reduction in DTR is not unique to global warming. Spatially, both the instrumental and reconstructed DTR show different modes on the NETP and SETP. Composite analysis and superposed epoch analysis reveal the linkage between El Niño‐Southern Oscillation (ENSO) and the DTR on the SETP. La Niña (El Niño) winters are generally followed by the stronger (weaker) early Indian summer monsoon so that may lead to the decrease (increase) in May–June DTR. The positive correlation between Niño 3.4 index and the DTR on the SETP over the past two and a half centuries is accompanied by a stable response of the decreased DTR to La Niña events. These results suggest that ENSO is a crucial driver for the DTR variation on the SETP through oceanic‐atmospheric modulation. [ABSTRACT FROM AUTHOR]