A new approach for the synthesis of Prussian blue type nanoparticles containing nanocomposites in the form of beads or films, as well as their corresponding aqueous colloids, was developed by using a water-soluble alginate matrix as a template and as a stabilizing agent. This method consists of the step-by-step building of a cyanometallate network in the pores of Mⁿ⁺/alginate ionotropic gels in order to obtain a large range of nanocomposites containing cyano-bridged coordination polymer nanoparticles Mⁿ⁺/[M′(CN)_m]³⁻/alginate (where Mⁿ⁺ = Ni²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Eu³⁺ and M′ = Fe³⁺, Cr³⁺ (m = 6), Mo⁵⁺ (m = 8)). The nanocomposite beads and films, as well as the corresponding aqueous colloidal solutions, were studied by infrared (IR), UV/visible spectroscopy, and transmission electron microscopy (TEM) analyses, which reveal the presence of homogeneously dispersed uniformly sized cyano-bridged coordination polymer nanoparticles of 3–7 nm. These nanocomposite beads and films present superparamagnetic, spin-glass or paramagnetic behaviour depending on the nature of the metal ions used. In addition, the Eu³⁺-containing nanocomposites are room temperature optically active emitters displaying a characteristic ⁵D₀ → ⁷F_0–4 transition.