Fifty-nine steers were used in a randomized block design to examine the effects of supplemental protein source on finishing steer growth performance and carcass characteristics. The four supplemental N treatments were: 1) urea (urea control), 2) solvent-extracted soybean meal (SBM), 3) expeller-processed soybean meal (SoyPLUS®; SP), and 4) dried brewers grains (DBG). The diets were isonitrogenous (13% CP dry matter basis) and fed for 84 d. From d 85 to 194, all steers were fed the urea-supplemented diet. Steers supplemented with natural protein compared to urea consumed 13% more feed (P<0.01) and gained 11% faster but the effect was not significant (P=0.20). Feed efficiency was not altered by treatment (P>0.10). There were no differences (P>0.10) in steer performance among natural protein sources. Previous supplemental N source did not affect (P>0.10) growth performance of steers after 84 d. Feed cost of gain for the DBG-supplemented diet was the highest (P<0.05). Steers fed supplemental true protein had heavier carcasses, but the difference was not detected statistically (P>0.10). The combination of competitive cost of gain and heavier carcasses for steers supplemented with SBM and SP resulted in average returns per head $9.23 and $15.79 higher than for steers supplemented with urea.