Abstract This study aimed to determine effects of dietary supplementation of Saccharomyces yeast postbiotics (celluTEIN, Puretein Bioscience, LLC, Minneapolis, MN) on nursery pig muscle fiber cross-sectional area (CSA), isoform, and succinate dehydrogenase (SDH) staining intensity. Newly weaned pigs [n = 32; 16 barrows and 16 gilts; 21 d of age, body weight (BW) 6.05 ± 0.24 kg] were stratified by BW within sex, placed in individual pens, and randomly assigned to one of two dietary treatments within each two-pig strata. Dietary treatments included pigs supplemented 0 (CON; n = 16) or 175 g/ton (CT; n = 16) of celluTEIN to their standard three-phase nursery ration. Pigs were harvested at d 35 of feeding and the longissimus lumborum muscle was collected for mean SDH staining intensity and immunohistochemistry analyses. Data were analyzed as a completely randomized design with pig as the experimental unit and treatment as the fixed effect. Statistical significance was determined at P ≤ 0.05. There were no treatment effects on type I and type IIB muscle fiber CSA (P > 0.14), but CT pigs had greater type IIA and type IIX muscle fiber CSA compared with CON (P < 0.05). When not analyzed by fiber isoform, pigs fed CT had a greater (P = 0.02) average muscle fiber CSA than CON pigs. There were no treatment effects on muscle fiber type percentage or SDH intensity of all fiber types (P ≥ 0.23); however, when calculated as a muscle fiber CSA ratio, CT type IIA fibers had a smaller ratio (darker staining/µm2) than CON type IIA fibers (P = 0.05). There were no treatment effects for type I, IIX, and IIB ratios (P > 0.21). In conclusion, supplementing Saccharomyces yeast postbiotics during the nursery period increased muscle fiber hypertrophy and type IIA fiber oxidative capacity.