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This study aimed to assess the impacts of dietary supplementation with passionfruit (<i>Passiflora edulis</i>) peel powder (PSPP) on the growth, immune response, and expression of immune and antioxidant-related genes in Nile tilapia (<i>Oreochromis niloticus</i>) maintained in a biofloc system. Fish were fed basal diets supplemented with different doses of PSPP at 10 g kg⁻¹ (PSPP10), 20 g kg⁻¹ (PSPP20), 40 g kg⁻¹ (PSPP40), and 80 g kg⁻¹ (PSPP80). The basal diet, without PSPP-supplementation, was used as a control at 0 g kg⁻¹ (PSPP0). We observed that the dietary supplementation groups fed different levels of PSPP exhibited no substantial difference or only slight increases in growth performance and immunological response in Nile tilapia (<i>p</i> > 0.05), whereas fish fed diets supplemented with PSPP at concentrations of 10 g kg⁻¹, 20 g kg⁻¹, and 40 g kg⁻¹ had significantly higher mRNA transcripts (approximately 1.5–4.5 fold) of immune (<i>il-1</i>, <i>il-8</i>, and <i>lbp</i>) and antioxidant (<i>gst-α</i>, <i>gpx</i>, and <i>gsr</i>) gene expressions than fish in the control treatment group (0 g kg⁻¹). These findings suggest that dietary supplementation with PSPP may effectively stimulate the immune and antioxidant defense system and may function as feed additives in Nile tilapia cultured in a biofloc system.