The uptake of COVID-19 booster vaccination among older adults in China is suboptimal. Here, we report the results of a parallel-group cluster-randomized controlled trial evaluating the efficacy of promoting COVID-19 booster vaccination among grandparents (≥60 years) through a health education intervention delivered to their grandchildren (aged ≥16 years) in a Chinese cohort (Chinese Clinical Trial Registry: ChiCTR2200063240 ). The primary outcome was the uptake rate of COVID-19 booster dose among grandparents. Secondary outcomes include grandparents' attitude and intention to get a COVID-19 booster dose. A total of 202 college students were randomized 1:1 to either the intervention arm of web-based health education and 14 daily reminders (n = 188 grandparents) or control arm (n = 187 grandparents) and reported their grandparents' COVID-19 booster vaccination status at baseline and 21 days. Grandparents in the intervention arm were more likely to receive COVID-19 booster vaccination compared to control cohort (intervention, 30.6%; control, 16.9%; risk ratio = 2.00 (95% CI, 1.09 to 3.66)). Grandparents in the intervention arm also had greater attitude change (β = 0.28 (95% CI, 0.04 to 0.52)) and intention change (β = 0.32 (95% CI, 0.12 to 0.52)) to receive a COVID-19 booster dose. Our results show that an educational intervention targeting college students increased COVID-19 booster vaccination uptake among grandparents in China.