Context: Age-related cognitive decline has become an important public health issue. Tai Chi may be an effective intervention to protect the cognitive ability of healthy adults, but its effects are uncertain. This study systematically evaluated the protective effects of Tai Chi on healthy adults' cognitive ability. Evidence acquisition: A systematic review of prospective controlled trials comparing Tai Chi with usual physical activities for cognitive ability maintenance among healthy adults was conducted. Seven electronic databases were searched from their inception to December 31, 2013. Data analysis and bias risk evaluation were conducted in 2014. Evidence synthesis: Nine studies, including four RCTs and five non-randomized controlled trials, with 632 participants were identified. Global cognitive function was measured using the Mini-Mental State Examination, Mattis Dementia Rating Scale (MDRS), or event-related potential 300 in three studies; attention was measured by the MDRS attention score, hands and feet alternating movement time, or response time in three studies; learning and memory were assessed by MDRS memory score, Wechsler Adult Intelligence Scale, or Auditory Verbal Learning Test in three studies; emotion and perception were measured using arm stability and mental rotation in one study; and execution was measured by Trail Making Test, Stroop Test, and Clock Drawing Test in four studies. Tai Chi showed a positive effect on most outcomes of various cognitive realms. Conclusions: Compared with usual physical activities, Tai Chi shows potential protective effects on healthy adults' cognitive ability. Large RCTs with more rigorous designs are needed to fully evaluate and confirm its potential benefits.