The Lancang-Mekong River Basin (LMRB) is vital to ~ 70 million people in Mainland Southeast Asia, crucially reliant on primary freshwater-dependent sectors like agriculture and fishery. Hence, assessing water resources availability over the LMRB is important for local, regional, and global sustainability. As the ratio of precipitation to potential evapotranspiration, the Aridity Index (AI) reflects dryness and wetness patterns, which are naturally dependent on large-scale climate teleconnections, such as the Pacific Decadal Oscillation (PDO). The present study assessed the effects of these teleconnections on AI variability in the LMRB, using gridded (25º × 25º) daily precipitation and mean Surface Air Temperature (SAT) time series extracted from historical (1952–2005) NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP). The Mann-Kendall non-parametric test and Spearman’s rank correlation were applied to identify trends in AI and its relationships with teleconnections, respectively. Key findings include: (i) the LMRB was predominantly humid, except during January, February, March, and December; (ii) AI showed a drying trend across most time scales; (iii) the PDO strongly affected AI during the dry season (Nov-May), while (iv) the East Pacific/North Pacific (EP/NP) during the wet season (Jun-Oct); and (v) the PDO was also influential annually (Nov-Oct) and in most months. These results provide critical insights for enhancing water security and advancing the UN’s 2030 Sustainable Development Agenda.