A feeding, digestibility, and balance trial was performed to determine the effect of the incorporation of increasing amounts of soybean protein concentrate (SPC) to a basal acorn diet at a fixed energy intake on the rate of growth, protein deposition (PD), and carcass traits of heavy Iberian (IB) pigs. Sixteen purebred IB barrows (103.9 ± 0.9 kg initial BW) were used. The pigs were individually confined and allocated to 4 groups of 4 pigs each. Whole acorns were provided with slight restriction to minimize acorn kernel (AK) remains. Water was freely available. The incorporation of SPC raised the available CP from 49.1 g/kg DM (1.91 g total Lys/kg DM) in AK up to 135 g/kg DM (7.24 g total Lys/kg DM). After 72 d of trial, the pigs achieved 134.4 ± 1.3 kg BW and were then slaughtered. Average daily BW gain increased with increasing dietary CP (linear, P < 0.001; quadratic, P < 0.01), with a plateau at dietary CP concentrations (provided by AK and SPC) beyond 72.2 and 100 g/kg DM, respectively, for which an ADG of 449 g was achieved. There was a linear and quadratic effect of dietary CP and Lys level on the apparent digestibility of CP (linear, P < 0.001; quadratic, P < 0.05) and on the efficiencies of utilization of total N (retained N/N intake; linear, P < 0.001; quadratic, P < 0.001) and N apparently absorbed (retained N/apparent digestible N; linear, P < 0.05; quadratic, P = 0.067). Protein deposition ranged between −4.59 g/d in pigs not provided with SPC to 61.4 g/d in pigs fed the greatest level of SPC inclusion. The lowest daily intake of total Lys resulting in maximum PD (69.5 g/d) at the fixed level of energy intake observed (2.36 × ME maintenance requirements) was 21.0 g. Hot carcass and cold carcass weights, but not carcass yield, increased when feeding the daily ration containing 100 g SPC and leveled off thereafter (linear, P < 0.01; quadratic, P < 0.01). An average increase of 8.3% and 8.7% was obtained, respectively. The provision of SPC had no effect on back fat thickness. Primal cuts, but not yield (except for loin), increased (sirloin: linear, P < 0.05; loin: linear, P < 0.001; quadratic, P < 0.05; ham: linear, P < 0.05) or showed a tendency to increase (butt lean: linear, P = 0.068; quadratic, P = 0.077; ribs: quadratic, P = 0.067; shoulder: quadratic, P = 0.064) with the provision of dietary protein. No effect of the level of incorporation of SPC to the diet was observed on CP, total fat, total ash, and energy contents or on relative fatty acid profile of loin and sirloin.