The additive manufacturing (AM) technique that is most popular for the fabrication of thermoplastic polymer structures is fused deposition modeling (FDM). However, the FDM printer’s limited bed size restricts the size of the prints, making it challenging to fabricate large and complex shapes. A practical solution to this issue is to split the CAD model and join the parts using a suitable joining technique. In this study, the specimens were 3D printed using M30 acrylonitrile butadiene styrene (ABS) thermoplastic polymer by varying four distinct printing parameters, namely (a) infill pattern, (b) infill angle, (c) infill density, and (d) layer thickness. These specimens were then joined to form a lap joint using an ultrasonic welding machine by varying two distinct welding parameters, specifically (a) welding pressure and (b) welding time. Then, tensile tests were performed in two phases of experiments. In the first phase, specimens were printed with constant printing parameters and welded by varying one welding parameter at a time while keeping the other parameter constant. The welding parameters that provided optimal joint strength were identified for the second phase of experiments. In this phase, specimens were printed with varying printing parameters and welded under the previously identified optimal welding conditions. Finally, the results of the tensile tests were analyzed using the Taguchi method to look into the effect of various welding and printing parameters on the tensile strength of the lap joint of the specimens. The results of the study found that a welding pressure and welding time of 3 bars and 3000 ms, respectively, and the printing parameters of hexagram infill pattern, 60% infill density, 45° infill angle, and a layer thickness of 0.127 mm, provided the maximum tensile strength of the welded joint.