The in-situ immobilization of cobalt (Co) or nickel (Ni) nanoparticles (NPs) in a polymer-derived silicon nitride (Si3N4) matrix is firstly reported. It is accomplished by the coordination of a polysilazane with metal chlorides (CoCl2 or NiCl2) followed by the subsequent pyrolysis of the metal:organosilicon polymer coordination complex in the temperature range 700–1000 °C in flowing ammonia. By coupling X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) on powders, it has been shown that the phase composition and crystallinity (NPs and matrix) as well as the crystal size of the NPs can be finely tuned through the synthesis parameters including the type of metals, the Si:metal ratio, the reaction time, the self-crosslinking ability of the considered polysilazane in the presence of the metal salts and the heat-treatment temperature. Then, the micro-/mesoporosity of the Si3N4 matrix has been developed using a nano-casting approach to exploit the catalytic activity of NPs towards the hydrolysis of sodium borohydride (NaBH4). Hence, compounds with a matrix delivering BET specific surface areas (SSA) as high as 921.5 m2 g and supporting the NPs have been investigated to evaluate their potential for catalysis.