OBJECTIVES: The causal relationship between slow gait speed and poor cognition is uncertain due to potential reverse causality. This study aimed to examine the reciprocal relationship between gait speed and global as well as domain-specific cognitive functions in older adults. DESIGN: Prospective cohort study (2013-2022) including 4 repeated measures. SETTINGS AND PARTICIPANTS: Community-dwelling and non-demented adults aged ≥65 recruited at baseline. METHODS: Biennial evaluations included assessments of 4-m usual walking speed, global cognition using the Taiwanese version of the Montreal Cognitive Assessment (MoCA), and cognitive domains (memory, executive function, verbal fluency, and attention) using a battery of neuropsychological tests. Cross-lagged models and linear mixed models were used to examine the bidirectional association between gait speed and cognitive functions with adjustment for potential confounders. RESULTS: At baseline, 511 participants were enrolled, and 459 individuals with a mean age of 74.5 years were ultimately analyzed. A 1-SD decrease in gait speed was associated with a 0.07-0.09 SD decrease in subsequent global cognition (βˆ CONCLUSIONS AND IMPLICATIONS: This study found a reciprocal association between gait speed and global or domain-specific cognition. Regularly screening gait speed and cognitive function enables the early detection of declines in physical function and cognition.