Objective: There is growing interest in t'ai chi, but little research has addressed whether t'ai chi is effective in older people using wheelchairs for mobilization. The aim of this study was to compare the effects of seated t'ai chi exercise and usual standard activities on mood states and self-efficacy in older people living in a long-term care facility and using wheelchairs for mobilization. Design: Randomized controlled trial (trial registration no. ACTRN12613000029796). Setting: One long-term-care facility in Taiwan. Participants: Sixty participants were randomly assigned by a computer-generated random sequence to a t'ai chi group (n = 30) or a usual exercise and entertainment activities group (n = 30). Intervention: Seated t'ai chi exercise for 40 minutes three times a week for 26 weeks was provided. Main outcome measures: Mood states (Profile of Mood States Short Form [POMS-SF]) and self-efficacy (Self-Efficacy for Exercise [SEE]). Results: At week 26, participants in the t'ai chi group reported significantly lower mood states on the fatigue-inertia dimension of the POMS-SF (mean score ± standard deviation, 3.56 ± 3.71) than did the control group (mean score, 7.16 ± 6.36) (F [1, 58] = 7.15; p < 0.05). The t'ai chi group recorded significantly higher SEE levels (mean, 35.66 ± 36.83) than did those in the control group (mean, 15.30 ± 26.43) (F [1, 58] = 6.05; p < 0.05). Conclusion: The findings highlight the importance of t'ai chi for a reduction in the fatigue-inertia mood state and an increase in self-efficacy for older people using wheelchairs.