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This paper presents a novel method for the ultrasonically assisted insertion of metallic z-pins into thermoplastic composites. Mechanical and microstructural investigations were carried out on glass fiber-reinforced polyamide and polypropylene specimens. The insertion of steel pins into thermoplastic composites led to microstructural changes that differ significantly from the known microstructure of z-pinned thermoset fiber-reinforced plastics. Optical microscopy showed an absence of notable fiber waviness and resin-rich zones around each pin. Instead, the fibers were predominantly deflected in the through-thickness direction by the high insertion forces arising during pin penetration. To gain an initial insight on the resulting properties of the z-pin/thermoplastic interface, the mechanical properties of z-pinned thermoplastic composites under mode I loading were investigated using pullout tests. For reference, the pullout behavior of thermoset carbon fiber-reinforced plastic specimens, reinforced with steel pins was determined too. Due to the poor bonding and lack of friction between the pin and laminate, the determined traction loads of the thermoplastic specimens are well below typical values achieved from pin pullout in thermoset laminates.