Web technologies facilitating location-independent participation and broad knowledge sharing are critically important for open science and participative management and decision-making. Therefore, it is crucial to enable hydrological models to run in a web environment. In this study, a representational state transfer (REST) interface is proposed for the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) model. The web service interface (named HMS-REST) was built based on the Java API and Jython script system of HEC-HMS. HMS-REST allows users to retrieve and write model inputs and outputs and perform model execution. These functionalities were evaluated via a case study of pseudoensemble flood forecasting using a data set of six historical flood events from the Chuanchang watershed, southeastern China. Satisfactory performance was achieved in pseudoensemble flood forecasting in terms of the peak flow, total flood volume, peak flow timing and overall hydrograph fitting. This case study indicates that HMS-REST and HEC-HMS can be effectively integrated and demonstrates that the use of HEC-HMS with a web-based interface can facilitate pragmatic applications of the HEC-HMS model in the fields of model calibration, flood simulation and forecasting, and participatory decision-making.