<p><i>Tagetes patula</i> L. is a prominent plant that is frequently used for its ornamental value. Marigolds are sources of various compounds of great interest to agricultural, pharmaceutical, and nutritional industries. Microalgae can be applied as a biostimulant as they have multifunctional properties in agriculture, namely facilitating nutrient uptake, improving crop performance and physiological status, and thus increasing tolerance to abiotic stress. This study evaluated the effect of applying <i>Chlorella vulgaris</i> as a plant biostimulant on the growth and flowering of <i>T.</i> <i>patula</i> plants. Microalgae suspensions were applied to the substrate at concentrations of 0.5, 2.0 and 5.0 g dry weight (DW) L⁻¹. A set of plants without the application of microalgae was used as control. The use of microalgae had a positive impact on plant growth. Specifically, <i>C. vulgaris</i> application resulted in greater height, while the highest concentration (5.0 g DW L⁻¹) resulted in higher fresh weight. Microalgae application significantly promoted the earlier flowering of marigolds. The macronutrient and micronutrient values were statistically similar between the control plants and those treated with microalgae. When considering only the treated plants, there was a significant correlation between the concentration of <i>C. vulgaris</i> and the concentrations of N, P, Mn and Cu in the shoots of <i>T.</i> <i>patula</i> plants. Plants receiving higher concentrations of <i>C. vulgaris</i> exhibited higher levels of these nutrients. These results highlight the potential use of <i>Chlorella vulgaris</i> as an effective biostimulant for enhancing the growth, flowering, and nutrient uptake of <i>T.</i> <i>patula</i>, offering valuable applications for improving ornamental plant production and, ultimately, agricultural sustainability.</p>