The present work aimed to examine the tensile and flexural behaviors of biocomposites reinforced with continuous plant fibers, utilizing a range of polylactic acid (PLA) matrix materials and varying fiber content. These biocomposites were fabricated using an in situ-impregnated fused filament fabrication (FFF) technique. The study incorporated three different PLA matrix materials, namely PLA, PLA-Matte (PLA-Ma), and PLA-ST, each with distinct mechanical properties. The effect of different linear densities of continuous ramie yarns on the biocomposites was also investigated. The results show that adding continuous ramie yarn significantly enhances both the tensile and flexural strengths, as well as the modulus, of the matrixes. Furthermore, there was a positive correlation between the content of ramie yarn and the increases in strength and modulus. Moreover, the introduction of ramie yarns altered the fracture behavior of the biocomposites, shifting towards brittle fracture. This change significantly impacted the fracture toughness of the matrixes and resulted in a convergence of elongation at the point of breakage.