<p>Rising fuel security challenges can be addressed by utilizing potential and renewable energy feedstocks such as microalgae. This study focuses on biodiesel production from <i>Desmodesmus armatus</i> NCIM 5583 by optimizing biomass productivity and lipid content. Independent optimization experiments were performed to study key growth parameters, viz., light intensity, photoperiod, and NaNO₃ concentration. Growth conditions with 40 µmol/m²/s light intensity, 20 h L:04 h D photoperiod, and 0.25 g/L NaNO₃ concentration exhibited highest biomass productivity of 26.51 ± 1.1 mg/L d and lipid yield of 45.1 ± 0.16% (w/w). Further, CaO bionanocatalyst synthesized using waste eggshells were characterized using SEM, EDX, FT-IR, and XRD. The results from SEM and EDX confirmed the structural and elemental composition of bionanocatalyst with Ca and O as the main elements. XRD revealed the crystalline nature of CaO with a particle size of 28.74 nm. Lipid transesterification using the synthesized catalyst yielded 84 ± 0.2% (w/w) FAME and was analyzed using FT-IR, ¹H NMR, ¹³C NMR, and GC–MS. The results confirmed the presence of the methyl ester group, implying the conversion of microalgal lipids to biodiesel. The findings from this study demonstrate the potential of <i>D. armatus</i> for biodiesel production and the application of waste eggshells as transesterification catalysts.</p>