The aim of the study was to assess the efficacy of two dietary levels of 6-phytase in male ducks fed diets with reduced available phosphorus (P) over a 42-day period. A total of 320 ducks (Cherry Valley SM3) were divided into 4 groups, each consisting of 16 floor pens (replicates) of 5 birds. Birds were offered diets during the starter (days 1–14) and grower (days 15–42) phases that contained standard available P content (0.50 and 0.38%, respectively; control, T1), reduced available P content (0.30 and 0.20%, respectively; negative control, T2), or diets as in T2 supplemented with phytase at 1000 (T3) or 2000 (T4) phytase units (FTU)/kg diet. During the experiment, birds’ performance was recorded, and excreta were collected for nutrient digestibility determination. At the end of the trial, carcass traits, breast muscle quality, and tibia mineralisation were determined. Data (n = 16 per treatment) were analysed using one-way ANOVA, with P < 0.05 considered significant. Birds fed the T2 diet showed reduced body weight (907 vs. 947 g; P = 0.005) and daily gain (60.7 vs. 64.5 g; P = 0.043) during the starter phase. Supplementation with 6-phytase significantly improved nitrogen, calcium, P, and ash retention (2.5, 7.4, 11.1, 5.2%, respectively) with a dose-dependent effect on mineral utilisation. Increased tibia ash content in phytase-supplemented ducks further confirmed improved P bioavailability (P = 0.005). Carcass yield was unaffected; however, phytase supplementation reduced breast muscle redness and yellowness, potentially linked to decreased oxidative stress. Overall, phytase effectively mitigates the negative effects of dietary P deficiency in ducks, improving growth performance, nutrient utilisation, and selected aspects of meat quality, particularly in the early growth phase.