: The aim of this study was to investigate the association between relatives’ interpersonal functioning and patients’ recovery after severe traumatic brain injury (TBI) across one year in Switzerland. : This prospective, multi-center cohort study is comprised of 188 adult patients with severe TBI (Abbreviated Head Injury Score > 3) and their relatives. Patients and relatives were assessed 3, 6, and 12 months post-injury. : Interpersonal functioning (Patient Competency Rating Scale for Neurorehabilitation, PCRS-NR), Physical and Mental Health related Quality of Life (HRQoL, SF-12), and overall functioning (Glasgow Outcome Comma Scale Extended, GOSE). : Multilevel analyses showed that relatives’ interpersonal functioning was positively associated with a) patients’ mental HRQoL ( =.002; slope = 2.95; β =.24) independently of age, b) a moderation time*patients’ physical HRQoL among patients > 50 years of age ( <.045; slope = 2.63; β =.2) and c) patients' GOSE among younger individuals ( <.001; slope =.60; β =.23). : These findings show that health and overall functioning are linked with interpersonal dimensions. Thus, the interplay between relatives and patients with TBI needs to be further investigated.