Oxygen isotopes in tooth enamel vary with temperature and precipitation cycles during an organism’s development, aiding reconstructions of past environments. Enamel is typically sampled by micro-drilling to recover oxygen inputs from food and water consumed during tooth formation. This method has limited spatial resolution, yielding samples that integrate long formation times of unknown chronological age, thereby hampering the recovery of seasonal patterns. We circumvent this problem with Sensitive High Resolution Ion Microprobe (SHRIMP SI) measurements of oxygen isotope compositions (δ18O) on a scale of 15 - 30 μm, which can be related to daily increments to guide sequential weekly sampling over multiple years of enamel growth. Here we analyze four molars of late Pleistocene Sumatran orangutans from Sibrambang and Lida Ajer – the site that yielded the oldest insular Southeast Asian human remains. Concurrently forming teeth from the same Lida Ajer orangutan range from 15.7 - 20.1 ‰ and 15.1 - 19.9 ‰, supporting the biogenic fidelity of these palaeoclimate records. Values from two individuals from the nearby Sibrambang cave are similar (15.3 - 20.4 ‰, 14.7 - 20.8 ‰). Fossil values fall within the range of six modern Sumatran orangutan molars (11.3 - 21.2 ‰), suggesting similar environmental conditions. Isotopic variation in primate enamel formed over multiple years is substantial, and complex seasonal trends are evident. Future research may help to establish whether climate variation was a significant force in the evolution and dispersal of the human genus (Homo) in the Indo-Pacific region. The research is funded by the Australian Academy of Science Regional Collaborations Program and the Australian Research Council.