Objective: Given the prognostic significance of gait speed, there is insufficient evidence about possible functional determinants of gait speed in patients with heart failure with reduced ejection fraction (HFrEF). Therefore, the objective of this study was to investigate the functional determinants of gait speed in patients with HFrEF. Methods: Fifty-nine patients with HFrEF participated in this cross-sectional study. Demographic and clinical characteristics were recorded. The gait speed was determined with a 4-meter walking test. Dyspnea perception was assessed with the modified medical research council (mMRC) scale. Functional capacity was evaluated with a 6-minute walk test (6MWT). The five times sit-to-stand (5-STS) test and the Berg Balance Scale (BBS) were used to measure functional mobility and balance. Physical activity was evaluated with the International Physical Activity Questionnaire (IPAQ) Short-Form. Results: Gait speed was correlated with age (r=-0.368, p=0.004), NYHA functional class (r=-0.438, p=0.001), mMRC score (r=-0.422, p=0.001), 6MWT (r=0.650, p<0.001), 5STS (r=-0.506, p<0.001), BBS (r=0.586, p<0.001), IPAQ (r=0.305, p=0.019) and IPAQ-Sitting time (r=-0.327, p=0.011). On multiple linear regression analysis, the 6MWT distance and BBS were independent determinants of the usual gait speed in patients with HFrEF, accounting for 44.4% of the variance. Conclusion: This study indicates that functional capacity and balance are independent functional determinants of gait speed in patients with HFrEF.