Previous research indicates that finishing steer ADG and G:F increases linearly with increasing dietary Zn-amino acid complex supplementation. The objective of this study was to determine the influence of supplemental Zn-amino acid complex on growth performance and carcass characteristics of finishing steers fed Optaflexx (OPT; ractopamine hydrochloride) for 0, 28, or 42 d before harvest. This study was organized as 2 groups (Grp) of steers fed concurrently, for 91 (Grp 1) or 84 d (Grp 2). A total of 324 steers (463 ± 23.4 kg) were fed a corn-based finishing diet supplemented with 60 mg Zn/kg diet DM (as ZnSO₄). Steers were blocked by weight (6 steers per pen) and assigned to receive either 0 (CON) or 60 mg supplemental Zn/kg DM from a Zn amino-acid complex (ZnAA; n = 27 pens per treatment). Receiving ZnAA for 49 (Exp 1) or 42 d (Exp 2) before start of OPT feeding had no effect on growth of steers (P ≤ 0.30). Forty-two days before harvest, pens were equally assigned within CON or ZnAA treatments to receive OPT at 300 mg⋅steer^–1⋅d^–1 for 0, 28 (28-OPT) or 42 d (42-OPT) before harvest, creating 6 final treatments (n = 9 pens per treatment). All steers within an experiment were harvested on the same day. Pen was the experimental unit, and the SAS model included the fixed effects of ZnAA, OPT, and block nested within group, and the random effect of pen. Optaflexx supplementation increased carcass-adjusted ADG (P = 0.004), final BW (P = 0.004), HCW (P = 0.004), and ribeye area (P = 0.007). There was an effect of ZnAA within 28-OPT and 42-OPT where carcass-adjusted ADG (P ≤ 0.10), final BW (P ≤ 0.05), and HCW (P ≤ 0.05) were greater in ZnAA supplemented vs. CON steers. However, when steers did not receive OPT there was no effect of ZnAA on final BW (P = 0.78), ADG (P = 0.98), or HCW (P = 0.78). In conclusion, there appears to be a synergistic effect of ZnAA on OPT-induced cattle growth, as supplementing 60 mg Zn/kg DM from ZnAA to cattle fed OPT improved overall growth and HCW.