Adjustment disorder (AD) is one of the most frequent mental health conditions after stressful life experiences in the medical setting. The diagnosis has been conceptually redefined in International Classification of Diseases (ICD-11) and now includes specific symptoms of preoccupations and failure to adapt. The current study assesses the prevalence of self-reported ICD-11 AD among organ transplantation patients and their relatives, explores the association of patients’ demographic-, transplant-, and health-related characteristics and ICD-11 AD symptoms, and evaluates the role of social support in the post- transplant context. A total of N = 140 patient-relative dyads were examined cross-sectionally. Hierarchical linear regression analyses were conducted to explore potential predictive factors of AD. The results revealed an AD prevalence of 10.7% among patients and 16.4% among relatives at an average of 13.5 years after the transplantation. The time that had passed since the transplantation was unrelated to AD symptom severity. Women tended to be at a higher risk in both groups. Somatic issues were predictive for AD only among patients and social support was predictive mainly among relatives. The results suggest that ICD-11 AD is a relevant diagnosis after organ transplantations for patients and relatives and its specific symptom clusters may provide important information for developing intervention strategies.