Background: Limitations in motor function experienced by people with multiple sclerosis (PwMS) may contribute to physical deconditioning. Here, we explored relationships between measurements of central nervous system function obtained with transcranial magnetic stimulation (TMS), walking and balance motor function, and aerobic fitness in PwMS. Methods: This ancillary study uses baseline data from an ongoing randomized controlled trial of PwMS. TMS was applied to determine corticospinal excitability and inhibition via active motor threshold (aMT) and cortical silent period (CSP) duration for the tibialis anterior (TA) and first dorsal interosseous (FDI) of the stronger limb. Motor function assessments included the Timed 25-foot Walk Test (T25-FW) and Mini-Balance Evaluation Systems Test (Mini-BESTest). Aerobic fitness (V̇O2-peak) was determined using a maximal exercise test. Independent-sample, unequal variance t-tests examined differences in motor function and fitness between participants with (TA+) and without (TA−) a response to TMS in the TA muscle. Spearman’s correlations evaluated relationships among measures. As preliminary research, comparisons and correlations yielding medium-to-large effect sizes were noted (Cohen’s d or rho ≥ 0.5). Results: Baseline data from 16 participants was analyzed (age: 56±10 years; T25-FW range: 4.3-180 s; Mini-BESTest range: 2-22). Compared to TA+ participants (n=9), TA− participants (n=7) demonstrated slower T25-FW times (44.5±73.5 s vs. 8.7±4.6 s, d=0.75, p=0.29) and lower V̇O2-peaks (10.6±2.2 ml/kg/min vs 15.9±3.6 ml/kg/min, d=−1.3, p<0.01). Among TA+ participants, higher TA aMT and longer CSP duration were associated with faster T25-FW performance (rho≥|0.52|, p≤0.15) and greater Mini-BESTest scores (rho≥|0.65|, p≤0.06). There were no relationships between FDI aMT and CSP with T25-FW or Mini-BESTest performance (n=14; rho≤|0.12|, p≥0.70). Both TA and FDI aMT were correlated with V̇O2-peak (rho≥|0.80|, p≤0.01). Discussion: Corticospinal excitability and inhibition for the TA muscle may be uniquely associated with walking and balance motor function, while corticospinal excitability associations with aerobic fitness may be independent of the muscle tested.