Background: This study aimed to investigate the effectiveness of a 6-month home-based high-intensity interval training (HIIT) intervention to improve peak oxygen consumption (V̇O2peak) and lactate threshold (LT) in older adults. Methods: Two hundred thirty-three healthy older adults (60–84 years; 54% females) were randomly assigned to either 6-month, thrice-weekly home-based HIIT (once-weekly circuit training and twice-weekly interval training) or a passive control group. Exercise sessions were monitored using a Polar watch and a logbook for objective and subjective data, respectively, and guided by a personal coach. The outcomes were assessed using a modified Balke protocol combining V̇O2peak and LT measures. General linear regression models assessed between-group differences in change and within-group changes for each outcome. Results: There was a significant between-group difference in the pre-to-post change in V̇O2peak (difference: 1.8 [1.2; 2.3] mL/kg/min; exercise: +1.4 [1.0; 1.7] mL/kg/min [~5%]; control: −0.4 [−0.8; −0.0] mL/kg/min [approximately −1.5%]; effect size [ES]: 0.35). Compared with controls, the exercise group had lower blood lactate concentration (−0.7 [−0.9; −0.4] mmol/L, ES: 0.61), % of peak heart rate (−4.4 [−5.7; −3.0], ES: 0.64), and % of V̇O2peak (−4.5 [−6.1; −2.9], ES: 0.60) at the intensity corresponding to preintervention LT and achieved a higher treadmill stage (% incline) at LT (0.6 [0.3; 0.8]; ES: 0.47), following the intervention. Conclusion: This study highlights the effectiveness of a home-based HIIT intervention as an accessible and equipment-minimal strategy to induce clinically meaningful improvements in cardiorespiratory fitness in older adults. Over 6 months, the exercise group showed larger improvements in all outcomes compared with the control group. Notably, the LT outcome exhibited a more pronounced magnitude of change than V̇O2peak. © 2024 The Author(s). Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.