Hydrogen peroxide (H2O2) is generated as a byproduct via enzymatic decomposition reactions of biological molecules; therefore, H2O2-sensing devices that function in wide pH ranges have been developed. Prussian blue (PB) is one of the most promising materials for constructing electrochemical H2O2-sensing devices; however, PB undergoes hydrolytic decomposition via the N-coordinated Fe(III) of the H2O2-sensing active Fe(II)-CN-Fe(III) moieties even in the weak basic condition of pH 8. In this study, it is anticipated that the homogeneously distributed Ni atoms in the partially Ni-displaced PB analog (Fe-Ni PBA, Fe1−xNix[Fe(CN)6]0.67·nH2O) effectively block serious structural damage from hydrolytic decomposition. Fe-Ni PBA nanoparticles (NPs) have been successfully prepared in all of the metal composition ratios between x = 0 and 1 and are stably dispersible into water by a surface modification method using [Fe(CN)6]4−. The spin-coated thin films composed of the Fe-Ni PBA NPs on ITO substrates function as an electrochemical H2O2 sensor even in the weak basic condition of pH 8, showing a linearity of the catalytic currents within the H2O2 concentrations between 2 × 10−4 and 5 × 10−3 mol/dm3.