Friction Stir Welding is a solid state joining process that has been shown to successfully join Mg alloys. However the presence of FSW Mg AZ31B in a lap joint configuration in the literature is largely underrepresented even though the use of this weld geometry is prevalent in the automotive and aerospace industries. FSW of Mg AZ31B in a lap joint configuration is presented in this research. Mg AZ31B was experimentally joined using an array of rotation and traverse speeds with a modified Tri-FluteTM pin design. It was found that the strongest weld joints occurred at the high rotation rate of 2000 rpm and the high traverse speed of 21 ipm. Macrosection analysis showed that all of the welds contained joint line remnants that were detrimental to the welds at the lower rotation speed of 1500 rpm. The severity of the joint line remnant increased the presence of the hooking defect. The results of this work help to fill in the gaps where there is otherwise little to no information regarding the effects of process parameters on weld quality for the FSW of Mg AZ31B in the lap joint configuration.