Background: Postural alignment impairments can be observed after acquired brain injury (ABI) and may be more prevalent in people with greater mobility impairment. However, investigations of postural alignment are limited by the lack of an evaluated method of assessment. This study examined postural alignment differences between healthy adults and adults with ABI, and evaluated repeatability of a new kinematic method for measuring postural alignment. Methods: Observational study of 14 adults with ABI who were non-ambulant at eight weeks post-injury, and 14 healthy adults. 3D kinematic data were recorded using motion capture technology in sitting and standing, on two separate days. A new Postural Alignment and Dispersion (PAD) score quantified average body segment displacements from the base of support (BoS) in the transverse plane. The two groups’ PAD scores were compared. Repeatability of the PAD score between days was evaluated using intraclass correlation coefficients (ICCs). Major findings: Greater postural alignment deviations were observed after ABI, including marked deficits in frontal and sagittal planes. ICCs ranged moderate (0.614) to excellent (0.945). Wide confidence intervals (CIs) for some ICCs indicated that repeatability of PAD scores could not be assured for all sitting and standing conditions. Conclusions: Postural alignment impairments occur in multiple planes after ABI and can be pronounced in people with severe mobility impairment. Evidence is needed to determine if addressing postural alignment impairments affects functional progress after ABI. PAD score measurement requires further evaluation to determine how repeatability can be optimised.