Embedded freeform 3D printing is an emerging 3D printing technique, where a hydrogel-based viscous medium provides support for extruded ink filaments during the printing process, allowing for omnidirectional printing of soft materials. One of the advantages of printing in a support bath is that the composition of the liquid medium can be modified to induce various chemical reactions in printed objects for solidification or functionalization of extruded ink filaments. Herein, we introduced a hybridization process to a 3D freeform printing system to achieve the direct fabrication of nanoparticle-reinforced composite hydrogels. In most composite hydrogel 3D printing systems, particles are preloaded in the ink prior to printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in-situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the rheological behavior of pure ink materials. Therefore, we successfully fabricated hyaluronic acid (HAc)-calcium phosphate (CaP) nanocomposite scaffolds through 3D freeform printing of HAc, coupled with in-situ precipitation of CaP. Phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the support bath for inducing the in-situ precipitation during 3D printing. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability as well as biological performance compared to pure HAc. Moreover, multi-material printing of composites of different CaP content was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries via the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.