Abstract: In this study, a bridge seismic monitoring system was developed based on smartphones connected together in the same project as a sensor network using a cloud server for easy upload and access to data. Thus, a decentralized structural seismic monitoring network was established using easy-to-install smartphones as both sensing cells and intelligence neurons. A long-term field test of this system was per-formed on Japan’s Takamatsu bridge. Smartphones with the monitoring application were deployed at six different locations inside the bridge’s concrete box girder. The system was connected to a cloud server for real-time data uploading and remote control. Inconsistent sampling rates and faulty sensor readings were identified to be two major problems with the use of the proposed system; however, correction methods were also proposed accordingly. A few seismic acceleration response records of the Takamatsu bridge were captured during the more than one-year operation period. Dynamic properties extracted from this system were compared with those of reference seismometers to verify its viability and accuracy.