This study evaluated the effects of completely replacing dietary inorganic phosphate (Pi) with phytase in a high-phytate diet (>0.30% phytate phosphorus [phytate P]). Male Ross 308 broilers (n = 390) were distributed in a completely randomised design to one of three experimental treatments, with 10 replicate pens per treatment and 13 birds per pen in three feeding phases: starter (days 0–10), grower (days 11–23), and finisher (days 24–35). The positive control (PC) diet was based on corn, broken rice, soybean, wheat, rice bran, and rapeseed meal, supplemented with Pi from monocalcium phosphate. The two experimental diets (P1000 and P1500) were formulated to be nutritionally equivalent but contained no added Pi. Instead, they were supplemented with 1000 or 1500 phytase units (FTU)/kg, with corresponding reductions in Ca (0.17 and 0.24%) and available phosphorus (0.20 and 0.28%), respectively. A negative control was not included for animal welfare reasons. Feed conversion ratio improved during the finisher phase (days 24–35) and over the entire experimental period (days 0–35) in the P1000 and P1500 groups compared to the PC group (P < 0.05). Carcass yield on day 35 did not differ between the phytase-supplemented groups and the PC group. Overall, replacing Pi with 1000 or 1500 FTU/kg can compensate or even improve growth performance and carcass yield, without adversely affecting bone strength or serum P and Ca concentrations in broilers.