*

Magnetic order has been proposed from a variety of defects including vacancies, anti-sites, and grain boundaries, which are relevant in numerous spintronics applications. Nevertheless, the origin of magnetism remains unelucidated, mainly due to the lack of structural analysis. We herein demonstrate the escalation of ferromagnetism in a vanadium (V)-doped WSe₂ monolayer by tailoring Se-vacancy complexes via post heat-treatment. Structural analysis of atomic defects was systematically performed using scanning transmission electron microscopy (STEM). Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-treatment. The V – Se vacancy pairing is a key to promote ferromagnetism with additional Se vacancies around V via spin-flip by electron transfer from Se-vacancy complex to V atom, predicted from density-functional-theory (DFT) calculations. The elevated magnetic moment with heat-treatment was estimated by combining the Se-vacancy population extracted from STEM with DFT calculations, which is well corroborated by magnetic domain analysis. Our approach towards nano-defect engineering paves a way to promote weak magnetic order in diluted magnetic semiconductors for renovating spintronics with two-dimensional layered materials.