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Soybean [ Glycine max (L.) Merr.] seed protein is negatively correlated with seed oil and often with yield. Our goal was to examine the basis for these correlations at a quantitative genetic locus (QTL) level. Seventy‐six F 5 –derived recombinant inbred lines (RILs) from the mating of the high‐protein (480 g protein per kg seed) G. max accession PI 437088A with the high‐yield cultivar Asgrow A3733 (420 g kg −1 seed protein content) were evaluated in six irrigation treatments (i.e., 100, 80, 60, 40, 20, and 0% replacement of weekly evapotranspiration loss) of a two‐replicate experiment conducted for 2 yr. The RILs were genotyped with 329 random amplified polymorphic DNAs (RAPDs), 103 simple sequence repeats (SSRs), and four other markers, creating a 2943‐centimorgan (cM) genetic map of 35 linkage group (LGs) that, on the basis of SSR homology, aligned with the 20 known soybean LGs. The phenotypic regression of RIL protein and oil on yield revealed respective linear coefficients of −2.6 and +1.6 percentage points per kg ha −1 (a protein/oil exchange ratio of −1.6). A seed protein, oil, and yield QTL mapped close to RAPD marker OPAW13a in a small LG‐I interval that was flanked by the SSR markers Satt496 and Satt239. The additive effects of the PI 437088A allele on seed protein, oil, and yield were a respective +1.0 and −0.6 percentage points (a protein/oil ratio of −1.6) and −154 kg ha −1 Given that the genetic‐based protein/oil exchange ratio of 1.6 is smaller than the 2.0 calorific‐based oil/protein ratio, one might expect the remaining 0.4 units of carbon and/or energy to be made available for other seed dry matter. However, yield almost invariably falls when seed protein is genetically enhanced at the expense of seed oil, suggesting that protein synthesis and its deposition in the seed is energetically more costly to the plant than is commonly assumed.