Irrigated agriculture dependent on runoff from the Tianshan Mountains underpins socioeconomic development in the arid regions of Central Asia. However, ongoing warming and agricultural expansion profoundly alter local water supply and demand. Whether future mountain water supplies can meet rising irrigation demand remains unclear, leaving a critical gap in assessing water security in one of the world's most vulnerable agricultural frontiers. Here, we quantified the future risks to irrigation water security in the Tianshan Mountains by comparing changes in mountain runoff and downstream irrigation demand between the historical (1991-2020) and future (2071-2100) periods. The results indicate that the risk of irrigation water scarcity in the northern slope of the Tianshan Mountains is projected to increase under future warming scenarios due to the growing imbalance between mountain runoff and downstream irrigation demand. The percentage of unmet irrigation demand in the Aibi Lake River System and Mid-branch River System of the north slope is projected to increase by 24%-32% and 14%-20%, respectively, in the future period compared to the historical period. In contrast, the risk in the southern slope of the Tianshan Mountains is expected to be mitigated, as projected increases in mountain runoff in the Weigan River System and Kaidu River System exceed rising irrigation demand by an average of 0.06 and 0.28 km3, respectively. To better reconcile the spatial distribution of irrigation demand and water resources, we recommend shifting 20% of the sown area in the Aibi Lake River System and 10% in the Mid-branch River System to the south slope.