Abstract <p>The effect of agar concentration <i>C</i>_A on the colloidal chemical properties of complex fish gelatin–agar hydrogels was studied using DSC, SEM, and 3D rheometry. The hydrogels were formed at constant gelatin content (10%) over a biopolymers ratio (weight/weight) <i>Z</i> ranging from 0.02 to 0.10 g_A/g_CFG. The addition of agar was shown to accelerate the gelation of fish gelatin by approximately 2 times. Gelatin increases the gelation temperature but does not affect the melting point of the agar component of the gel. Agar increases the melting heat by 13–14 times without affecting the melting point of the gelatin component of the gel. The complex hydrogel is characterized by a compact structure with a reduced mesh size compared to the gels of the individual biopolymer components. The microstructure of the gel consists of a cellular gelatin network reinforced by fibers of agar network. The strength and elasticity of the complex gel increase significantly with increasing <i>C</i>_A, while syneresis decreases. The optimal biopolymer ratio of <i>Z</i> = 0.08 g_A/g_CFG was determined at which a fish gelatin–agar hydrogel exhibits elastic-strength characteristics comparable to those of gelatin from porcine skin, and is also resistant to melting and syneresis during storage at room temperature.</p>