Phytase hydrolyzes phytate rendering phosphorus available for intestinal absorption, while systemic neutralization of fibroblast growth factor 23 (FGF-23), using anti-FGF-23 antibody, has been shown to increase phosphate retention. Hence, neutralization of FGF-23 should be additive with phytase in reducing dietary non-phytate phosphorus (nPP) needs in chickens fed plant-based diets rich in phytic acid. This study was designed to test the additive effects of maternally derived anti-FGF-23 antibody and dietary phytase on the performance of chicks fed a low nPP diet from one to 14 d. Single Comb White Leghorn laying hens were vaccinated with either an adjuvant control or a synthetic FGF-23 peptide (GMNPPPYS). Chicks from vaccinated hens with control or anti-FGF-23 maternal antibodies were fed either a diet containing 0.2% nPP and 0.9% calcium with or without 500 unit phytase per kg of diet (2 × 2 factorial with main effects of antibody type and phytase addition, n = 15 pens of chicks/treatment). A significant interaction between dietary phytase and maternally derived anti-FGF-23 antibody on growth and feed efficiency was observed (P ≤ 0.05), in which chicks receiving either phytase or maternally derived anti-FGF-23 antibody had improved body weight gain (21 or 15%, respectively) and feed efficiency (16 or 18%, respectively) as compared to chicks with control antibody and not fed phytase. Both phytase and maternally derived anti-FGF-23 antibody independently increased (P ≤ 0.05) plasma phosphate (11 and 11%, respectively) and percent tibiotarsus ash (13 and 11%, respectively). Significant main effects and the lack of an interaction supported an additive effect of phytase and anti-FGF-23 antibody on plasma phosphate and percent tibiotarsus ash. Feeding phytase to chicks fed 0.2% nPP increased plasma FGF-23 levels by 22% (P ≤ 0.05); however, no effects of anti-FGF-23 antibody on plasma FGF-23 levels were observed. In conclusion, dietary phytase and presence of anti-FGF-23 antibody have an additive effect on plasma phosphate and tibiotarsus ash in chicks fed low nPP diets. Data support that phytase and anti-FGF-23 antibody increase phosphate utilization by different mechanisms.