Immune stimulants are an alternative to antibiotic use and contribute to disease prevention in aquaculture. The effect of dietary administration of kappa carrageenan (Kc), extracted from the red algae Hypnea musciformis, in Nile tilapia (Oreochromis niloticus) was assessed by monitoring weight gain after a 15-day feeding trial. Immunostimulation was inferred by the relative expression of transferrin, interleukin 1β (IL-1β), and growth hormone (GH) in the spleen after 24 h and 15 days of daily administration. The toxic activity of Kc was evaluated in brine shrimp (Artemia salina) nauplii. No significant toxic effects of Kc were observed in A. salina at any dose studied. A positive tendency in growth rate and fish survival values was observed when Kc was administered. Correspondingly, GH, transferrin and IL-1 β levels at day 15 post-treatment were higher in the spleens of fish fed with Kc at 5 g kg−1 relative to non-Kc-treated control fish. Feeding Kc extract from H. musciformis to the fish improved nonspecific immunity parameters and increased survival and growth, but further research, including longer-termed studies, should be conducted before recommendation of Kc supplementation in tilapia diets at commercial scale.