Twenty clinically healthy and regularly trained horses, 10 Sella Italiana (Jumper) and 10 Thoroughbreds, were randomly divided into two subgroups. The first subgroup received a dietary supplement Omega Horses 70 ml/day for 30 days (experimental group); the second subgroup received no dietary supplement (control group). All horses were subjected to a simulated race to test their performance levels. The same race was performed on both groups at the end of experimental period. On blood samples, collected before and after the first test (T0–T0pe), every 7 days for a month (T1–T2–T3–T4) and after the second test (T4pe), Prothrombin Time (PT), Activated Partial Thromboplastin Time (APTT), Fibrinogen Concentration (Fb) and platelet aggregation were assessed. The application of two-way repeated measures analysis of variance (ANOVA) identified a significant effect of time (4 weeks monitoring) on PT only in Jumpers, on APTT and Fb both in Jumpers and Thoroughbred. A statistically significant effect of treatment was observed during the 4 weeks of monitoring on PT, APTT and Fb while no significant change was observed on platelet aggregation. Both experimental groups showed higher PT and APTT values and lower Fb values than control groups. Moreover, a statistically significant effect of treatment was observed on PT and Fb in T4–T4pe periods in both breeds. This study highlights the effects of omega 3 dietary supplementation on horse’s clotting parameters providing useful information to improve athletic horse’s management.