A steel structure footbridge characterized by its flexibility and long span is taken as a case. The designs of the structure and tuned mass damper (TMD) systems are introduced. In situ measurements are conducted after construction and after the installation of TMD devices. Then, the modal properties and vibration responses under various pedestrian load conditions with or without TMD devices are obtained. The human-induced vibration properties are analyzed based on the acceleration peak and acceleration root mean square (RMS) value. The results indicate the following: The vibration of the steel footbridge is dominated by the first vertical mode when one end of the bridge is designed as a sliding connection, and the torsional and horizontal vibrations are relatively small. There is no obvious difference in the vibration properties of a footbridge after TMD devices are installed. The distributed TMD produces good vibration tuned effects for the various excitation conditions with reduction ratios of 35%-70%. Moreover, the damping ratio increases over four times the value without TMD, which is very small before installation. The damping is large at the earlier stage of the free attenuation response and decreases with decreasing vibration amplitude. Lastly, the TMD′s efficiency in vibration reduction is insensitive to variation of the TMD damping ratio.