The profitability of growing soybean (Glycine max (L.) Merr.) is largely determined by the successful elimination of weeds and the weather conditions, the adverse effect of which can be minimized by the use of biostimulants. This study aims to evaluate the effect of several herbicides and biostimulants on the seed yield and contents of protein, fat, amino acids, and fatty acids in soybean seeds. The results demonstrate that the yield and quality of soybean seeds were most beneficially affected by the use of prosulfocarb immediately after sowing in combination with a subsequent foliar application of bentazone and imazamox with an adjuvant containing methyl oleate and fatty alcohol. This treatment ensured the highest seed yield (3.32 t ha−1), the highest contents in seeds of protein (342.4 g kg−1 DM), phenylalanine (15.65 g kg−1 DM), leucine (23.54 g kg−1 DM), and most of the endogenous amino acids. All herbicide treatments increased the contents of tryptophan, serine, and glutamic acid compared to the seeds without herbicide treatment. The study results indicate that soybean responded positively to all biostimulants, as indicated by a higher seed yield (from 5.3% to 11.3%), plant height, and contents of tryptophan, serine, glutamic acid, cysteine, tyrosine, and C18:1n9c + C18:1n9t acids in the seeds.