Objective: Thoracic radiotherapy (TRT) may result in toxicities that are associated with performance declines and poor quality of life (QOL) for patients and their family caregivers. The purpose of this randomized controlled trial was to establish feasibility and preliminary efficacy of a dyadic yoga (DY) intervention as a supportive care strategy. Methods: Patients with stage I to III non-small cell lung or esophageal cancer undergoing TRT and their caregivers (N = 26 dyads) were randomized to a 15-session DY or a waitlist control (WLC) group. Prior to TRT and randomization, both groups completed measures of QOL (SF-36) and depressive symptoms (CES-D). Patients also completed the 6-minute walk test (6MWT). Dyads were reassessed on the last day of TRT and 3 months later. Results: A priori feasibility criteria were met regarding consent (68%), adherence (80%), and retention (81%) rates. Controlling for relevant covariates, multilevel modeling analyses revealed significant clinical improvements for patients in the DY group compared with the WLC group for the 6MWT (means: DY = 473 m vs WLC = 397 m, d = 1.19) and SF-36 physical function (means: DY = 38.77 vs WLC = 30.88; d = .66) and social function (means: DY = 45.24 vs WLC = 39.09; d = .44) across the follow-up period. Caregivers in the DY group reported marginally clinically significant improvements in SF-36 vitality (means: DY = 53.05 vs WLC = 48.84; d = .39) and role performance (means: DY = 52.78 vs WLC = 48.59; d = .51) relative to those in the WLC group. Conclusions: This novel supportive care program appears to be feasible and beneficial for patients undergoing TRT and their caregivers. A larger efficacy trial with a more stringent control group is warranted.