Additive manufacturing (AM) of thermoplastic parts is a common technique for prototypes, small batches, and mass customization products. A widely used AM process is the fused deposition modeling (FDM), which generates parts with an inhomogeneous volume structure, because it is build up line by line and layer by layer. An industrial established joining technology is the laser transmission welding (LTW), e.g. for joining injection molded parts in the automotive sector. For this technique, the transmissivity of one joining partner has a high influence on the resulting weld seam quality and the welding process itself. In order to use LTW for joining AM parts, the influence of transmissivity and scattering behavior of AM parts were investigated. The optical properties were analyzed with spectroscopy and shear tensile tests were performed with welded samples to enhance the knowledge about the relationship between the FDM process, the optical behavior, and the weld seam strength.