Context: Current tools for sideline assessment of balance following a concussion may not be sufficiently sensitive to identify impairments, which may place athletes at risk for future injury. Quantitative field-expedient balance assessments are becoming increasingly accessible in sports medicine and may improve sensitivity to enable clinicians to more readily detect these subtle deficits. Objective: To determine the validity of the postural sway assessment on the Biodex BioSway™ compared with the gold standard NeuroCom Smart Equitest System. Design: Cross-sectional cohort study. Setting: Clinical research laboratory. Participants: Forty-nine healthy adults (29 females: 24.34 [2.45] y, height 163.65 [7.57] cm, mass 63.64 [7.94] kg; 20 males: 26.00 [3.70] y, height 180.11 [7.16] cm, mass 82.97 [12.78] kg). Intervention(s): The participants completed the modified clinical test of sensory interaction in balance on the Biodex BioSway™ with 2 additional conditions (head shake and firm surface; head shake and foam surface) and the Sensory Organization Test and Head Shake Sensory Organization Test on the NeuroCom Smart Equitest. Main Outcome Measures: Interclass correlation coefficient and Bland–Altman limits of agreement for Sway Index, equilibrium ratio, and area of 95% confidence ellipse. Results: Fair–good reliability (interclass correlation coefficient = .48–.65) was demonstrated for the stance conditions with eyes open on a firm surface. The Head Shake Sensory Interaction and Balance Test condition on a firm surface resulted in fair reliability (interclass correlation coefficient = .50–.59). The authors observed large ranges for limits of agreement across outcome measures, indicating that the systems should not be used interchangeably. Conclusions: The authors observed fair reliability between BioSway™ and NeuroCom, with better agreement between systems with the assessment of postural sway on firm/static surfaces. However, the agreement of these systems may improve by incorporating methods that mitigate the floor effect in an athletic population (eg, including a head shake condition). BioSway™ may provide a surrogate field-expedient measurement tool.