To examine the validity of the timed Up and Go (TUG) test as a measure of functional mobility in persons with multiple sclerosis (MS) by using a comprehensive framework based on construct validity (ie, convergent and divergent validity). Cross-sectional study. Hospital setting. Community-residing persons with MS (N=47). Not applicable. Main outcome measures included the TUG test, timed 25-foot walk test, 6-minute walk test, Multiple Sclerosis Walking Scale-12, Late-Life Function and Disability Instrument, posturography evaluation, Activities-specific Balance Confidence scale, Symbol Digits Modalities Test, Expanded Disability Status Scale, and the number of steps taken per day. The TUG test was strongly associated with other valid outcome measures of ambulatory mobility (Spearman rank correlation, rs=.71-.90) and disability status (rs=.80), moderately to strongly associated with balance confidence (rs=.66), and weakly associated with postural control (ie, balance) (rs=.31). The TUG test was moderately associated with cognitive processing speed (rs=.59), but not associated with other nonambulatory measures (ie, Late-Life Function and Disability Instrument-upper extremity function). Our findings support the validity of the TUG test as a measure of functional mobility. This warrants its inclusion in patients' assessment alongside other valid measures of functional mobility in both clinical and research practice in persons with MS.