Previous studies showed a positive effect of the DMRT3 “gait keeper” mutation on harness racing performance in Standardbreds, French-, and Nordic trotters. The mutation has also been shown to influence riding traits in multiple breeds. This study investigated the effect of the DMRT3 mutation on harness racing performance and riding traits in Finnhorses. Finnhorses used for harness racing (n = 180) and for riding (n = 59) were genotyped for the DMRT3 mutation. For the trotters the genotypes were evaluated for association with racing performance (number of starts, victories, placings, earnings, and race times). At 3–6 years of age the AA genotype was superior compared with the CA and CC genotypes. The AA horses had a significantly higher proportion of victories (P = 1.4×10−6) and placings (P = 4.1×10−7), better race times (P = 0.01), and earned more money (P = 0.009) compared with C-horses. For the Finnhorses used for riding the owners answered a questionnaire to score how well the horse performed the gaits walk, trot, and canter on a scale from 1 to 6. These scores were tested for association with the DMRT3 genotypes. Although AA horses were more successful as racehorses, the CC and CA horses appear more adapted for classical riding disciplines. The AA horses received significantly lower gait scores compared with C-horses for the majority of gaits. Except for rhythm in extended canter (P = 0.05), there were no significant differences between CA and CC horses. This study shows that there are different optimal genotypes for different disciplines and the DMRT3 mutation clearly influences gaits and performance in Finnhorses.