Objective Social support may have a positive impact on health outcomes for patients and caregivers, but the extent to which social support and health outcomes are interrelated for both is unknown. We examine the dyadic interrelationships between social support and health among cancer patients and their caregivers. Methods Lung and colorectal cancer (CRC) patient and caregiver dyadic data were obtained from the Cancer Care Outcomes Research and Surveillance Consortium. Patients and caregivers self‐reported sociodemographic, social support, and caregiving characteristics at 5 (n = 218 lung; n = 222 CRC) or 12 months post‐diagnosis (n = 198 lung; n = 290 CRC). Structural equation modeling was used to examine actor‐partner interdependence models (APIM) of lung and CRC dyads at 5 and 12 months post‐diagnosis. Results At 5 months post‐diagnosis, no interdependence between patient and caregiver social support was detected for CRC or lung dyads (all P > 0.05). At 12 months post diagnosis, no interdependence was detected for CRC dyads (all P > 0.70); lung dyads showed complete interdependence, indicating patient social support is associated with better caregiver self‐reported health (β = 0.15, P < 0.001), and caregiver social support is associated with better patient self‐reported health (β = 0.18, P < 0.001). Conclusion Social support has a positive impact on patient and caregiver perceived health across the cancer trajectory, and these effects may differ by cancer site and time. Future research and translational efforts are needed to identify effective ways to bolster both patient and caregiver social support and to determine critical moments for intervention.