The objective of this study was to assess the effect of a postbiotic additive from Aspergillus oryzae (AO) on milk production and body temperature in dairy cows exposed to heat stress. Forty-eight Holstein cows (105 ± 27 SD days in milk, 704 ± 23 kg body weight) were used in a completely randomized design for 36 d. Cows were randomly assigned to 1 of 4 treatments: 0 g/d (control; CTL), 3 g/d (low), 6 g/d (medium), and 18 g/d (high) of AO (Biozyme Inc., St. Joseph, MO). A 41% forage and 59% concentrate total mixed ration (18.1% CP, 33.0% NDF, 1.61 Mcal/kg NEL) was fed twice daily, and AO was top-dressed after each feeding. Cows experienced warm climate during June and July 2018 with heat abatement (i.e., fans and misters) from d 1 to 10 (period 1). On d 11 to 36, heat abatement was removed to increase heat stress (period 2). Feed ingredients and milk samples were collected and analyzed. Body temperature and respiration rate were monitored. Energy-corrected milk (ECM) was estimated from milk measurements. Treatment means were analyzed for each period using a mixed-effect model with polynomial contrasts for the increasing doses of the AO postbiotic using SAS. Assessment of temperature-humidity index showed that heat stress was mild in period 1 (74.6 ± 2.4 SD) whereas its intensity increased in period 2 (77.3 ± 4.2 SD). In period 1, AO decreased (P < 0.01) afternoon vaginal temperature following a cubic response. Additionally, AO tended to quadratically increase (P = 0.06) yields of milk and ECM by 3.2 and 3.6 kg/d (CTL = 40.9 and 37.7 kg/d). The AO postbiotic did not affect DMI, milk protein and fat yield, milk protein, fat content, and SCC. In period 2, AO quadratically increased (P < 0.01) yields of ECM, milk protein, and fat by 3.8, 0.08, and 0.16 kg/d (CTL = 36.4, 0.98, and 1.37 kg/d). Furthermore, AO linearly decreased SCC (P = 0.02) and morning vaginal temperature (P < 0.01) but did not affect DMI. In summary, the AO postbiotic increased milk yield parameters and reduced vaginal temperature in heat-stressed cows.