Dairy cows are during the transition period most susceptible to metabolic and infectious diseases, which adversely affect reproductive performance. Blackberry pomace, the waste product from blackberry processing, is rich in polyphenolic compounds that have anti-inflammatory and anti-oxidative properties. To evaluate the effect of supplementation of blackberry pomace during the transition period on health and reproductive performance of dairy cows, 24 multiparous dairy cows were fed either 0 (Control), 57, or 114 g/d of dried blackberry pomace as top dressing to the TMR from 28 d before to 28 d after calving. Blood samples were collected on approximately d 28, 21, 14, 7, 3, and 1 prepartum, while both blood and milk samples were collected on d 0, 1, 3, 7, 14, 21, and 28 postpartum. Upon completion of the study, serum samples were analyzed for BHB, glucose, FFA, BUN, calcium, and phosphorus concentrations. Reproductive data, including days open and number of times bred before confirmed pregnant, was collected. All data were analyzed using PROC MIXED and PROC GLIMMIX in SAS version 9.4. Fixed effects were blackberry pomace supplementation rate and breed and for repeated data within cows, time and the interaction of time with treatment. Compared with Control, supplementation with 114 g/d decreased the number of days until first heat (33 ± 5d vs. 48 ± 7d; P = 0.02), first breeding (58 ± 3d vs. 68 ± 8d; P = 0.04), and days open (73 ± 9d vs. 122 ± 22d; P = 0.002) and tended to improve pregnancy rate from first breeding (78 ± 17% vs. 16 ± 15%; P = 0.06). No significant group differences were observed for disease incidence (P = 0.34), feed intake (P = 0.58), milk yield (P = 0.36), or concentrations of serum for BHB (P = 0.31), glucose (P = 0.0525), FFA (P = 0.74), BUN (P = 0.65), calcium (P = 0.46), and phosphorus (P = 0.102). In conclusion, supplementation of blackberry pomace at 114 g/d during the transition phase may improve health and reproductive performance of dairy cows.