With soybean [Glycine max (L.) Merr.] seed cost increasing in Alberta, understanding economic optimum plant density (EOPD) could help growers save on input expenses. A study was conducted at two irrigated locations in southern Alberta (Bow Island and Lethbridge), in three growing seasons (2014–2016), using two maturity group (MG) 00 soybean genotypes, two row spacings (RS; narrow, 17.5 cm; wide, 35 cm), and three seeding densities (SD; 30, 50 and 80 seeds·m⁻²). Exponential plant density–yield relationships were used to estimate EOPD. The earlier MG 00.4 genotype compensated yield at lower plant density (39 vs. 43 plants·m⁻²) and emergence (74% vs. 80%) than the later MG 00.8 genotype. The EOPD gaps between environments, genotypes, and RS were minimal (from 1–3 plants·m⁻²), resulting in only 1.3%–2.0% differences in grain yield (37–56 kg·ha⁻¹), and gross revenue at EOPD ($16–24·ha⁻¹). The overall EOPD estimate was 46 plants·m⁻², regardless of environment, genotype or RS. The study highlighted the difference between agronomic production and profit maximization in choosing an optimum plant density, and the need to establish a seeding density calculator for irrigated soybean in southern Alberta.