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The nature of multidecadal to centennial variability of the Asian monsoon is largely unknown, hindering our understanding of how the modern precipitation regime compares to preindustrial variability, as well as how the Asian monsoon may change in coming decades. Here we use the sediment record from a closed-basin lake in southern Tibet, Ngamring Tso, along with other published paleoclimatic records, to assess summer monsoon precipitation variability over the 20th century and during the late Holocene (4100 cal yr BP to present). The first principal component of the Ngamring Tso grain size record, which is highly correlated with median grain size, correlates significantly with observed summer (June–September) precipitation. From CE 1940 to 2007, grain size decreased with increasing summer precipitation and increased with decreasing summer precipitation. Satellite images of Ngamring Tso reveal larger lake area in the 1970s and 2000s and smaller lake area in the 1980s and 1990s, suggesting monsoon precipitation-induced changes in lake depth or area likely govern grain size variability. In the context of the last 4100 yr, prolonged periods of relatively weak summer monsoon precipitation occurred from 2800–2600 cal yr BP, 2500–2300 cal yr BP, and 1600–400 cal yr BP. A trend toward increased summer precipitation began around 1000 cal yr BP, with above-average summer precipitation from 400 cal yr BP to present, peaking between 200–100 cal yr BP (CE 1750–1850). Dry and wet periods are coincident with dry and wet periods in other south-central Tibetan lake sediment records that reflect precipitation or moisture balance, and with regional proxies of the ISM and EASM, indicating south-central Tibet is influenced by both monsoon subsystems. However, western Tibet lake sediment records do not covary with the Ngamring Tso record, suggesting the spatial variability in Tibet precipitation that occurs on interannual timescales today may also characterize precipitation on multidecadal and centennial timescales during the late Holocene. 20th century precipitation variability in southern Tibet falls within the range of natural variability in the last 4100 yr, and does not show a clear trend of increasing precipitation as projected by models. Instead, it appears that poorly understood multidecadal to centennial internal modes of monsoon variability remained influential throughout the last 4100 yr. Thus, substantial multidecadal to centennial-scale hydroclimatic variability will complicate our ability to project and prepare for anthropogenic changes in the region's monsoon precipitation.