The late Oligocene is an important deep-time analog for understanding future changes in the strength of the East Asian monsoon: it represents a climate warmer than today, yet follows the nascent uplift of the Tibetan–Himalayan orogeny during the Eocene Epoch. Here we quantify monsoon strength based on new oxygen isotope measurements on cellulose (δ¹⁸O_cell) extracted from modern and fossil wood from southern China. Tree-ring δ¹⁸O_cell values have previously been used to track Holocene climate variations in East Asia, as δ¹⁸O_cell values are primarily controlled by meteoric water δ¹⁸O (δ¹⁸O_MW) and relative humidity. We find the δ¹⁸O_cell values measured on the modern samples (25.7 to 29.1‰ VSMOW) are consistent with other δ¹⁸O_cell records from trees growing in southern China under the present-day monsoon climate. However, fossil wood δ¹⁸O_cell values (21.0 to 24.1‰ VSMOW) are significantly lower than those from living trees in the region, and instead overlap with values from modern high latitudes and high elevations. We show that these low δ¹⁸O_cell values are best explained by much higher rainfall amounts in southern China during the late Oligocene, with monthly wet-season rainfall that may have been ~60% greater than today based on modern relationships. These data represent the first seasonal rainfall estimates for southern China during the late Oligocene and signify an intensification of the region's current monsoonal rainfall patterns. We speculate that significantly greater monsoon rainfall is therefore possible in the region under a warmer climate.