The aim of this study was to establish the concurrent validity and relative and absolute reliability, define the minimal detectable change, and evaluate the floor and ceiling effects of the Four Square Step Test (FSST) in ambulant persons with multiple sclerosis (pwMS). Twenty-five pwMS performed the FSST on two occasions, 8.1±4.1 days apart. During the first testing, session participants also reported their fall history, performed the Berg Balance Scale, Dynamic Gait Index, and completed the Activities-Specific Balance Confidence Scale. Performance on the FSST was significantly (P<0.001) and strongly associated with performance on the Berg Balance Scale (rs=-0.84), Dynamic Gait Index (rs=-0.81), and Activities-Specific Balance Confidence Scale (rs=-0.78). Relative reliability of the FSST was excellent (ICC2,1=0.922). The minimal detectable change estimate for the FSST was 4.6 s. The FSST is a valid and reliable measure of dynamic standing balance in ambulant pwMS. However, because a substantial change (43%) is required to demonstrate a real change in individual performance, the FSST is unlikely to be sensitive in detecting longitudinal change in dynamic standing balance.