The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser. The microstructure, distribution of microhardness, and tensile properties of the base metal (BM) and welded joint were investigated. The results showed that the fusion zone of the welded joint had the highest microhardness due to the formation of 100% martensite. A finely mixed microstructure of martensite, ferrite, and austenite was formed in the heat-affected zone, and there was no softened zone in this area. The tensile test results indicated that the ultimate tensile strength and yield strength were higher for the joint than for BM. The joint efficiency was approximately 100%. All samples of the welded joint failed at the location of BM during tensile deformation. The fracture surfaces of the BM and welded joint were mainly ductile fractures. The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01–1 s−1, while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s−1 due to changes in the dislocation movement mechanisms. The uniform elongation of the BM and welded joint decreased with increasing strain rate.