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Polylactic acid (PLA) is distinguished for its high biocompatibility and elastic modulus comparable to that of human bone, which makes the polymer an ideal biodegradable matrix for implants. However, the mechanical properties and radiopacity of PLA are insufficient for such application; in particular, its radiopacity is too low to assess the position and biodegradation of implanted devices by magnetic resonance imaging and computed tomography. Our study shows that the level of mechanical strength, radiopacity, and biodegradation uniformity can be increased by filling a PLA matrix with magnetic Fe-Fe3O4 nanoparticles. Such particles of average size 68 nm with an α-Fe rich core and a Fe3O4 shell (≈4%) were formed via electrical iron wire explosion in an oxygen-containing atmosphere and were filled into PLA via polymerization to obtain Fe-Fe3O4@PLA composites differing in Fe-Fe3O4 content. The properties of Fe-Fe3O4@PLA were analyzed on its 3D printed specimens. The analysis suggests that with a Fe-Fe3O4 mass faction of up to 10 wt %, the strength of the composite grows, and this prevents its fracture along printing defects. Increasing the Fe-Fe3O4 content to over 15 wt % causes the formation of nanoparticle agglomerates which act as effective stress concentrators and the large size of which reduces their mobility when the polymer matrix is involved in plastic flow. The presence of Fe-Fe3O4 nanoparticles greatly increases the radiopacity of the material and stimulates the growth of 3T3 fibroblast cells. The developed material is promising for 3D printing of bone scaffolds and screws. [ABSTRACT FROM AUTHOR]