High-intensity interval training (HIIT) is highly beneficial for health and fitness and is well tolerated. Treadmill-based HIIT normally includes running interspersed with walking. The purpose of this study was to compare ungraded running and graded walking HIIT on perceived exertion, affective valence, and enjoyment. Thirty-four active, healthy adults completed maximal testing and two 20-min HIIT trials alternating between 85% of VO2peak and a comfortable walking speed. Affective valence, enjoyment, and perceived exertion, both overall (ratings of perceived exertion [RPE]-O) and legs only (RPE-L), were measured. RPE-O and affective valence were similar between HIIT trials (p > .05), RPE-L was higher for walk HIIT (p < .05), and enjoyment was higher for run HIIT (p < .05). Findings indicate that both walk and run HIIT produce exertion, affective, and enjoyment responses that are positive and possibly supportive of exercise behavior. Walk HIIT may be desirable for individuals who are unable or do not want to run.