Objective: Cancer places a psychological burden on both patients and their relatives. Perceived social support influences the extent of psychological distress. Our aim was to investigate associations between positive support, detrimental interactions, depression and anxiety in patient-relative dyads in the initial period after diagnosis. Methods: Patients with a solid tumor and their relatives participated in this prospective, multicenter observational study. Participants answered validated measures including the Illness-specific Social Support Scale (SSUK-8), the Patient Health Questionnaire (PHQ-9) and the General Anxiety Disorder Scale (GAD-7). We analyzed cross-sectional data from the initial time following diagnosis with paired t-tests and actor-partner interdependence models. Results: A total of 347 dyads of patients (mean age 59.9 years) and their relatives (mean age 56.7 years) participated. Compared to their relatives, patients reported significantly higher levels of depression (patients: M = 6.31, SD = 4.94; relatives: M = 5.44, SD = 4.77) and lower levels of anxiety (patients: M = 4.40, SD = 4.10; relatives: M = 4.98, SD = 4.47) as well as more positive support (patients: M = 14.31, SD = 2.07; relatives: M = 12.46, SD = 3.29) and a lower frequency of detrimental interactions (patients: M = 3.21, SD = 2.97; relatives: M = 3.66, SD = 2.93). Intrapersonal effects: Positive support was associated with lower distress only for relatives, whereas detrimental interactions were associated with higher distress for both patients and relatives (all p < 0.05). Interpersonal effects: More positive support and fewer detrimental interactions experienced by relatives were associated with lower patient distress (all p < 0.05). Conclusions: Better support for relatives may not only reduce their own distress, but also patients' distress. Relatives experience similar levels of distress and poorer social support than patients.