Acute respiratory infections are a leading cause of morbidity and mortality among young children globally. The objective of this study was to evaluate the impact of the Cholera-Hospital-Based-Intervention-for-7-days (CHoBI7) handwashing with soap and water treatment mobile health (mHealth) program on respiratory illness among diarrhea patients and their household members in urban Dhaka, Bangladesh. A cluster-randomized controlled trial of the CHoBI7 mHealth program was conducted among diarrhea patient households in Dhaka, Bangladesh. Patients were randomized to three arms: standard recommendation on oral rehydration solution use, health facility delivery of CHoBI7 plus mHealth (weekly voice and text messages for 12 months) (no home visits), and health facility delivery of CHoBI7 plus two home visits and mHealth. Respiratory symptoms were assessed during monthly clinical surveillance over the 12-month surveillance period. Respiratory illness was defined as rapid breathing, difficulty breathing, wheezing, or coughing. Two thousand six hundred twenty-six participants in 769 households were randomly allocated to three arms: 849 participants to the standard message arm, 886 to the mHealth with no home visits arm, and 891 to the mHealth with two home visits arm. Compared with the standard message arm, participants in the mHealth with no home visits arm (Prevalence Ratio [PR]: 0.89 [95% CI: 0.80, 0.98]), and the mHealth with two home visits arm (PR: 0.89 [95% CI: 0.81, 0.99]) had significantly lower respiratory illness prevalence over the 12-month program period. Our findings demonstrate that the CHoBI7 mHealth program is effective in reducing respiratory illness among diarrhea patient households.