Aging is associated with decreased aerobic fitness and cardiac remodeling leading to increased risk for cardiovascular disease. High-intensity interval training (HIIT) on the treadmill has been reported to be more effective in ameliorating these risk factors compared with moderate-intensity continuous training (MICT) in patients with cardiometabolic disease. In older adults, however, weight-bearing activities are frequently limited due to musculoskeletal and balance problems. The purpose of this study was to examine the feasibility and safety of non-weight-bearing all-extremity HIIT in older adults. In addition, we tested the hypothesis that all-extremity HIIT will be more effective in improving aerobic fitness, cardiac function, and metabolic risk factors compared with all-extremity MICT. Fifty-one healthy sedentary older adults (age: 65 ± 1 years) were randomized to HIIT (n = 17), MICT (n = 18) or non-exercise control (CONT; n = 16). HIIT (4 × 4 min 90% of peak heart rate; HRpeak) and isocaloric MICT (70% of HRpeak) were performed on a non-weight-bearing all-extremity ergometer, 4 ×/week for 8 weeks under supervision. All-extremity HIIT was feasible in older adults and resulted in no adverse events. Aerobic fitness (peak oxygen consumption; VO2peak) and ejection fraction (echocardiography) improved by 11% (P < 0.0001) and 4% (P = 0.001), respectively in HIIT, while no changes were observed in MICT and CONT (P ≥ 0.1). Greater improvements in ejection fraction were associated with greater improvements in VO2peak (r = 0.57; P < 0.0001). Insulin resistance (homeostatic model assessment) decreased only in HIIT by 26% (P = 0.016). Diastolic function, body composition, glucose and lipids were unaffected (P ≥ 0.1). In conclusion, all-extremity HIIT is feasible and safe in older adults. HIIT, but not MICT, improved aerobic fitness, ejection fraction, and insulin resistance.