This study presents a novel method for three-dimensional (3D) printing of continuous fiber-reinforced thermoplastics (FRTPs) using in-nozzle impregnation based on fused-deposition modeling (FDM). The technique enables direct 3D fabrication without the use of molds, potentially becoming the next-generation standard for composite fabrication. A thermoplastic filament and continuous fibers are separately supplied to the 3D printer, with fibers impregnated with the filament within the heated nozzle before printing. Polylactic acid (PLA) was used as the matrix, while carbon fibers or twisted jute fibers were used as reinforcements. The resulting composites showed improved mechanical properties compared to conventional 3D-printed polymer-based composites. 3D printing allows the fabrication of complex 3D parts without expensive molds or tools, based on 3D computer-aided design (CAD) data. While several 3D printing systems are available, FDM-based printing using thermoplastics is particularly widespread due to its simplicity and potential applicability. However, the mechanical properties of FDM-printed parts are inherently poor due to the thermoplastic resins used. The study addresses this by developing a method to fabricate continuous fiber-reinforced composites, which significantly improves mechanical properties. Carbon-fiber composites exhibit excellent mechanical performance due to the high strength and stiffness of carbon fibers. Fiber-reinforced thermoplastic composites can be used in passenger automobiles for high recyclability and load-bearing capability. "Green" composites, based on natural fibers and biodegradable resins, are in high demand to meet regulatory requirements for recyclability. However, conventional fabrication methods require expensive facilities and equipment, hindering the wide application of composites. The study investigates the 3D printing of continuous FRTPs, combining fibers and resin in a nozzle based on FDM printing. The results show that the mechanical properties of 3D-printed continuous fiber composites are significantly improved compared to conventional composites. The tensile modulus and strength of 3D-printed carbon fiber-reinforced thermoplastics (CFRTPs) are 19.5 ± 2.08 GPa and 185.2 ± 24.6 MPa, respectively, which are 599% and 435% of those of the PLA specimen. The tensile strain-to-failure of CFRTP is decreased due to the low strain-to-failure characteristics of carbon fibers. The study also demonstrates that the mechanical properties of 3D-printed composites can be further improved by optimizing the fiber direction and volume fraction. The results show that the proposed method expands the applicability of 3D printing to the manufacture of load-bearing components, which cannot be realized by conventional 3D printing. The study highlights the potential of 3D printing for the fabrication of high-performance composites with improved mechanicalThis study presents a novel method for three-dimensional (3D) printing of continuous fiber-reinforced thermoplastics (FRTPs) using in-nozzle impregnation based on fused-deposition modeling (FDM). The technique enables direct 3D fabrication without the use of molds, potentially becoming the next-generation standard for composite fabrication. A thermoplastic filament and continuous fibers are separately supplied to the 3D printer, with fibers impregnated with the filament within the heated nozzle before printing. Polylactic acid (PLA) was used as the matrix, while carbon fibers or twisted jute fibers were used as reinforcements. The resulting composites showed improved mechanical properties compared to conventional 3D-printed polymer-based composites. 3D printing allows the fabrication of complex 3D parts without expensive molds or tools, based on 3D computer-aided design (CAD) data. While several 3D printing systems are available, FDM-based printing using thermoplastics is particularly widespread due to its simplicity and potential applicability. However, the mechanical properties of FDM-printed parts are inherently poor due to the thermoplastic resins used. The study addresses this by developing a method to fabricate continuous fiber-reinforced composites, which significantly improves mechanical properties. Carbon-fiber composites exhibit excellent mechanical performance due to the high strength and stiffness of carbon fibers. Fiber-reinforced thermoplastic composites can be used in passenger automobiles for high recyclability and load-bearing capability. "Green" composites, based on natural fibers and biodegradable resins, are in high demand to meet regulatory requirements for recyclability. However, conventional fabrication methods require expensive facilities and equipment, hindering the wide application of composites. The study investigates the 3D printing of continuous FRTPs, combining fibers and resin in a nozzle based on FDM printing. The results show that the mechanical properties of 3D-printed continuous fiber composites are significantly improved compared to conventional composites. The tensile modulus and strength of 3D-printed carbon fiber-reinforced thermoplastics (CFRTPs) are 19.5 ± 2.08 GPa and 185.2 ± 24.6 MPa, respectively, which are 599% and 435% of those of the PLA specimen. The tensile strain-to-failure of CFRTP is decreased due to the low strain-to-failure characteristics of carbon fibers. The study also demonstrates that the mechanical properties of 3D-printed composites can be further improved by optimizing the fiber direction and volume fraction. The results show that the proposed method expands the applicability of 3D printing to the manufacture of load-bearing components, which cannot be realized by conventional 3D printing. The study highlights the potential of 3D printing for the fabrication of high-performance composites with improved mechanical