Mountain regions play a vital role in providing water resources, yet their hydrological systems are under threat from by climate change, land-use change, and socio-economic pressures. Although Nature-based solutions (NbS) are increasingly being considered as a means of ensuring water security, evidence of their hydrological performance in mountain environments is inconsistent and limited. This study presents a critical scoping review of the evidence on the hydrological functions of NbS in mountain regions, examining how reported hydrological effects can inform their contribution to water security objectives. A total of 106 studies were identified and classified into three groups: forest-based interventions, wetlands, and water harvesting. The four most frequently assessed hydrological functions were flow regulation, water retention, groundwater recharge, and soil erosion control. The evidence was unevenly distributed geographically, with studies concentrated in a limited number of countries. Forest-based interventions showed relatively consistent evidence for flow regulation, whereas evidence for groundwater recharge was more variable. Wetland studies reported effects on flow regulation and water retention, but these varied between sites and assessment approaches. Water harvesting interventions, particularly terraces and infiltration trenches, were frequently linked with runoff reduction and soil erosion control. However, their evidence base was predominantly local in scale and less extensive for several hydrological functions. The reported effect ranges varied within the NbS groups, highlighting the context-dependent nature of their performance and limiting the possibility of direct comparisons between cases. Overall, the review identified recurrent hydrological functions and conditioning factors relevant to mountain NbS, while highlighting important gaps in baseline conditions, methodological assessment, and spatial coverage. These findings establish a foundation for improving the assessment of hydrological evidence in mountain regions and for developing frameworks that demonstrate the contribution of NbS to water security.