The Qazvin aquifer is a critically over-exploited resource and a primary water source for agriculture and industry in a semi-arid region of Iran. Accurate delineation of its geometry is therefore essential for sustainable management. This study aimed to develop a robust 3D hydrostratigraphic model of this aquifer by integrating a multi-disciplinary dataset, including topographic maps, geological data, geophysical surveys (445 VES points), and the locations of qanats and wells. Hydrostratigraphic interpretation was conducted using 88 well logs, while bedrock depth was revised at each borehole by synthesizing pumping tests, sedimentological records, and well performance data. The 3D geometry was subsequently evaluated using geophysical data as an initial framework and refined with well log analysis in both RockWorks (interpretive) and T-PROGS (geostatistical) software. Additionally, pumping tests and groundwater level changes were used to interpret aquifer-forming mechanisms and behavior. Results revealed a paleo-sedimentology influenced by seasonal flows in braided river channels, with a clear spatial distribution of coarser-grained sediments on the plain margins, fining towards the center. These findings demonstrate the expansion of adjacent alluvial fan environments converging towards the basin center. Model validation confirmed the accuracy and reliability of the final 3D stratigraphic framework, providing an essential tool for future groundwater management strategies.