Drought tolerance is an integral determinant of drought survival in trees; thus, an accurate and rapid assessment of drought tolerance can lead to improved prediction of forest responses to droughts. The osmometer method enables the rapid determination of the leaf water potential at turgor loss (π_tlp), a key parameter of drought tolerance, from the osmotic potential at full turgor (π_sat). However, despite its wide applications, there have been few validations in ever-wet tropical rainforests. Here, we assessed the efficacy of the osmometer method in dipterocarps, a dominant tree group in SE Asia in ever-wet Malaysia, and examined the linkage between π_tlp and distribution along a rainfall gradient. The π_tlp determined using conventional method was strongly and linearly correlated with the π_sat determined using an osmometer. The coefficients of our model were statistically identical to those previously represented, but with a slightly larger intercept (0.21 MPa). Species with more negative π_tlp were distributed in drier habitats, with a relatively larger variation in π_tlp for species confined to ever-wet climates than for those occurring in ever-wet and seasonally dry climates. Some leaf traits, individual leaf area and dry matter content, were associated with π_tlp, but these traits alone could not predict species distribution. We demonstrated the robustness of osmometer method in ever-wet tropical rainforest species and that π_tlp is associated with current distribution along large-scale moisture availabilities.