Poor seated balance negatively impacts the performance of activities of daily living in nonambulatory individuals with multiple sclerosis (MS) and is frequently a target of rehabilitation interventions. However, there is a lack of clinical measures of seated balance in nonambulatory individuals with MS, thus limiting evaluation of rehabilitation treatments. The aim of this investigation is to determine the reliability and concurrent validity of the Function in Sitting Test (FIST) as a measure of sitting balance in nonambulatory individuals with MS. Twenty nonambulatory individuals with MS [mean age±SD=56.8±10.9 years, women n=15 (75%), mean MS duration±SD=17.8±9.2 years, mean wheelchair usage duration±SD=5.9±4.7 years] underwent a FIST and posturography assessment. The FIST is a 14-item clinical functional assessment of sitting balance validated in adults with acute stroke. The seated posturography assessment involved participants sitting on a force platform without support for 30 s. On the basis of the center of pressure trajectory obtained from the force platform software, two force platform outcomes were quantified: sway area (mm) of the center of pressure and virtual time to contact to the functional boundary (seconds). Internal consistency reliability was assessed using Cronbach's coefficient-α. The test-retest reliability was evaluated using the intraclass correlation coefficient (ICC). Concurrent validity of the FIST was assessed by Spearman's rank correlation analysis. Cronbach's-α as an index of internal consistency of the FIST was 0.91. The test-retest reliability was found to be excellent (ICC=0.92). The FIST was significantly correlated with virtual time to contact to the functional boundary (ρ=0.487, P=0.02), but not with sway area (ρ=-0.267, P=0.25). The observations provide evidence that the FIST is a reliable and valid tool to assess seated postural control in nonambulatory individuals with MS.