We show that it is possible to tune the magnetic properties in MoS2 without any intentional magnetic dopants but by simply controlling the magnetically active defect sites. The layer thickness and defect-density-controlled MoS2 (2H-phase) nanosheets are obtained by annealing a wet-chemical precursor from 500 to 900 °C. High-resolution transmission electron microscopy images show the presence of planar and edge defects arising from abruptly terminating edges, bends, tears, and folding of nanosheets. In addition, Mo5+ and sulfur vacancy point defects are predominant, as evidenced from the electron paramagnetic resonance and X-ray photoelectron spectra. Nanosheets gradually become more crystalline and devoid of defects upon annealing, with MoS2 (900 °C) becoming >70% defect-free. The interlayer spacing decreases monotonically from ∼6.351 to 6.231 Å with annealing; however, the lattice parameter a increases from 3.107 to 3.172 Å. These structural changes induce a large local strain (∼9%) in MoS2 (500 °C) nanosheets compared to the small strain (∼3%) present in MoS2 (900 °C). Raman spectroscopy further manifests control on the defect density, with the intensity ratio of the defect-induced LA(M) mode (∼227 cm-1) relative to the E2g 1 and A1g modes decreasing monotonically with the annealing temperature. We demonstrate that magnetically active defect sites can be tuned to enhance the ferromagnetism. Magnetization (at 5 K) systematically varies with 1 order higher MS (=0.10 emu/g) found in MoS2 (500 °C) nanosheets compared to MoS2 (900 °C). Interestingly, temperature-dependent magnetization shows a ferromagnetic-like transition around 120 K, which becomes more pronounced in defect-rich MoS2. The ferromagnetic interaction is more likely due to a bound magnetic polaron made up of a spin 1/2 Mo5+ ion with trapped carriers present at the sulfur vacancies. These findings open a new way of exploring spintronic devices such as spin-field-effect switches, spin valves, magnetic sensors, and ultrathin high-density data storage devices using MoS2 2D nanosheets by controlling magnetically active defect sites. Copyright © 2019 American Chemical Society.