A potential biofilm forming and phenanthrene utilizing marine bacterium Pseudomonas mendocina NR802was isolated from Rushukulya, Odisha, East Coast of India. The effect of Ca²⁺and Mg²⁺on biofilm growthand phenanthrene degradation was evaluated. Among the various tested concentrations, 20 mM of Ca²⁺and Mg²⁺showed a significant enhancement in biofilm production by the bacterium. The SEM-EDAXstudy showed that the elemental composition of the biofilm varied significantly when grown in thepresence of Ca²⁺and Mg²⁺. The CSLM analysis of biofilms grown in the presence of 20 mM Ca²⁺andMg²⁺reveal the critical role of these ions on biofilm architectural parameters such as total biomass,biofilm thickness, roughness coefficient and surface to biovolume ratio. Ca²⁺was found to enhance theextracellular polymeric substances (EPS) production and phenanthrene degradation. Ca²⁺enhanced thebiofilm growth in a dose dependent manner, whereas Mg²⁺significantly increased the cell growth inbiofilm. More than 15% increase in phenanthrene degradation was observed when biofilm was grownin the presence of an additional 20 mM Ca²⁺. This study also supports the fundamental role of Ca²⁺ inbiofilm growth, architecture as well as biofilm-mediated pollutant degradation.