<p>The aquaculture industry is progressively prioritizing sustainable practices to address the expanding global food demand, and microalgae emerging as a key component. <i>Tetradesmus obliquus</i> is known for its high phenotypic plasticity in response to environmental changes. Nitrogen is a critical nutrient for algal growth. Understanding how different nitrogen sources and concentrations affect the growth and morphology of <i>T. obliquus</i> is essential for both exploring its phenotypic plasticity and optimizing its cultivation for practical applications. Therefore, this study investigated the effects of different nitrogen sources (i.e., nitrate, ammonia, and urea) at ten concentrations (5, 10, 20, 35, 50, 100, 180, 250&#xa0;mg L⁻¹) on the physiological and morphological characteristics of <i>T. obliquus</i>. Results showed under nitrogen-limiting conditions, algal growth was restricted, photosynthetic activity declined, and the colony size and volume were larger. At high ammonia levels (180, 250 mg L⁻¹), growth was inhibited, cell volume decreased, and unicell was the predominant morph. Conversely, high concentrations of urea led to a significant reduction in maximum and effective quantum yield of photosystem II (F_v/F_m and Φ_PSII), a substantial increase in cell volume, a transition in cell shape from spindle to ovoid, and biomass production exceeding that in nitrate or ammonia treatment at the same concentrations. These findings highlight that changes in growth and morphology are interdependent and dynamically respond to variations in nitrogen source and concentration. This study reveals how nitrogen dynamics affect <i>T. obliquus</i> growth and morphology, supporting future advances in sustainable algal biotechnology and environmental applications.</p>