Abstract: Laser powder bed fusion (LPBF) is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation. However, there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel. This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF. The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction, but it reduces the average size of the cellular microstructure from 0.60 to 0.35 μm. The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample, resulting in inadequate fusion and keyhole defects respectively. The optimal scanning speed for fabricating samples was determined to be 800 mm/s, which obtained the highest room temperature tensile strength and elongation, with the ultimate tensile strength measured at (1088.3 ± 2.0) MPa and the elongation of (16.76 ± 0.10)%. Furthermore, the mechanism of the evolution of surface morphology, defects, and energy input were clarified, and the relationship between cellular microstructure size and mechanical properties was also established.