It was previously reported that the oral administration of lactoferrin (LF) provides antimicrobial activity in animals against bacterial and viral infections and is involved in immunomodulatory properties. In this report, a hybrid strain of Oreochromis nilotica (male) × O. mossambicus (female) was fed homemade diets with or without supplementation with 10, 50, 100, and 150 mg bovine LF g−1 feed for 60 days. After dietary treatment, the experimental fish were infected with Streptococcus agalactiae. LF supplementation resulted in a significantly higher survival rate and suppression of bacterial growth at 24–96 h in the liver, spleen, kidneys, eyes, and gills. A reduction in peroxidase activity was followed by a similar reduction in the peroxidase content of leukocytes at 24 h as analyzed by spectrophotometry. Respiratory burst (RB) activity was detected regardless of the time at which LF was administered to fish in relation to the bacterial infection. A beneficial effect of LF on RBs was also detected before bacterial challenge (with 100 mg LF/g) and 24 h after bacterial challenge with 150 mg/g. The results obtained validate LF’s beneficial effects on RBs by phagocytes in tilapia, which was only shown at 48 h after supplementation with 100 mg bovine LF g−1 feed after the bacterial infection. Moreover, after oral administration of LF, it was detected in the mucosa of the small intestines of tilapia. In conclusion, we demonstrate a reduction in fish mortality after the oral administration of LF, and we examined its immunomodulatory properties in tilapia.