To address to burden of infectious diseases such as diarrhoea, the Vietnamese government has enacted the Law on Prevention and Control of Infectious Diseases (LPCIDs) since July 2008. However, no evaluation of the impact of the LPCID has been conducted. This study aims to evaluate the impact of the LPCID on diarrhoeal control for the 5 years following the implementation of LPCID in Vietnam. We used an interrupted time series design using a segmented regression analysis to estimate the ‘province-level’ impact of LPCID and then used random-effect meta-analysis to estimate the pooled effect sizes of the ‘country-level’ impact of LPCID on diarrhoeal control throughout Vietnam. The results show that the impacts varied by provinces. They were classified in four groups: ‘positive impact, positive impact without sustainability, possibly positive impact, no or negative impact’ of the LPCID. The meta-analysis indicated that the country-level impact of the LPCID became significant at 11 months after the LPCID took effect, with a decrease in level of diarrhoea of 9.7% (coefficient, −0.097; 95% CI: −19.1 to − 0.002) and a permanent downward trend of diarrhoea at a rate of 1.1% per month (coefficient, −0.011; 95% CI: −0.02 to − 0.003); whereas the trend in diarrhoea before the LPCID took effect was unchanging (coefficient, 0.002; 95% CI, 0–0.004). At 12, 24, 36, 48 and 60 months following the LPCID implementation date the levels of diarrhoea decreased by 10.9% (coefficient, −0.109; 95% CI: −0.203 to − 0.015), P < 0.01), 21.8% (coefficient, −0.218; 95% CI: −0.338 to − 0.098), P < 0.01), 31% (coefficient, −0.31; 95% CI: −0.474 to − 0.145), P < 0.01), 46.8% (coefficient, −0.468; 95% CI: −0.667 to − 0.27), P < 0.01), 48.2% (coefficient, −0.482; 95% CI: −0.708 to − 0.256), P < 0.01) respectively. The findings of this study reveal the effectiveness of the LPCID in reducing diarrhoea incidence in Vietnam. However, further studies should be conducted to better understanding the cost-effectiveness, acceptability, and sustainability of each component of the LPCID.