Background: Individuals with intellectual disabilities have poor levels of physical fitness due to low physical activity. Tai Chi appears to be associated with improvements in physical and mental conditions. However, studies of Tai Chi on individuals with intellectual disabilities are limited. Purpose: This study investigated the effects of Tai Chi and aerobic exercise training on physical fitness in teenagers with intellectual disabilities. Method: 66 students, aged 10 - 20 years, with intellectual disabilities ranging from profound to mild (IQ below 20 to 70) were invited to participate in this quasi-experimental study. Matches for age, gender and classification of intellectual disabilities were performed. Individuals with previously regular physical activity, low attendance at exercise sessions (<60%) or unqualified at Tai Chi performance were excluded. 50 participants completed 12 weeks of after-class intervention, which consisted of two Intervention groups, Tai Chi (TC, 12M & 2F) or aerobic exercise (AE, 14 M & 3F), and a Control group (CON, 16M & 3F). The Intervention groups had two 40-min exercise sessions per week and the Control group had no physical activities. A two-way repeated-measures analysis of variance (group × time) was used to test differences in physical fitness, and post-hoc Bonferroni tests were used to determine any significant differences between the groups. One-way ANCOVA with the pre-intervention rank as the covariate was also performed. Results: After 12-weeks intervention and compared with the Control group, body mass index, skinfold thickness, sit-ups and agility all improved significantly (p < 0.05), and vertical jump (TC, p < 0.01; AE, p < 0.05), balance (TC, p < 0.05) and push-ups (AE, p < 0.05) all increased significantly. Conclusion: Both Tai Chi and aerobic exercise have positive effects on physical fitness in teenagers with intellectual disabilities. Tai Chi could be a useful intervention for those with intellectual disabilities.