The fat-extracted microalgal biomass derived from the biofuel production has been suggested as a promising source of protein supplement in ruminant diets. Consequently, we performed an in vitro experiment to investigate the effects of supplementing microalgal protein precipitate material (APP) obtained via wet lipid extraction procedure. The APP was attained using dairy waste from Caine Dairy Research Center (Wellsville, UT) at Utah State University. The APP used in this study had low bacterial counts on blood agar, approximately 1.8 × 10² cfu/g. In addition, in a 24-h incubation assay with a suspension of bovine fecal contents, we observed neither an inhibitory nor a stimulatory effect of the APP on growth or survivability of Salmonella enterica serovar Typhimurium (8.65 ± 0.18 log₁₀ cfu/mL) or Escherichia coli O157:H7 (8.91 ± 0.11 log₁₀ cfu/mL). The APP contained 30.6% crude protein and 6.60% ether extract on a dry matter (DM) basis. The experiment was performed as a completely randomized design to test 4 dietary treatments in a typical dairy diet (n = 4): 1) no APP (control), 2) 1.2% APP, 3) 2.4% APP, and 3.6% APP in a DM basis. Although increasing APP in the diet tended to linearly decrease culture pH (P = 0.08), the culture pH was maintained at least 6.0 throughout culture incubation. Total VFA concentration was similar across dietary treatments. Molar proportion of acetate linearly increased by increasing APP inclusion (P = 0.03), while propionate portion did not change, resulting in no effect on acetate-to-propionate ratio. Increasing addition of APP linearly decreased molar proportion of isovalerate (P < 0.01). Concentration of ammonia-N averaged 7.32 mg/100 mL, and it was not influenced due to dietary treatments (P = 0.27). Additionally, APP supplementation did not affect methane production in the culture regardless of dietary concentration. Overall data in this in vitro study suggest that APP attained from dairy waste did not have any negative impacts when added up to 3.6% DM on ruminal fermentation profiles and show a potential to be a sustainable source of dietary protein in dairy diets.