Until now, Croatians had to cross land belonging to Bosnia and Herzegovina.
The 2.4km (1.5-mile) Peljesac bridge was built by China but largely funded by the European Union.
Celebrations took place throughout the day. Before traffic was allowed on the gleaming white structure, 250 runners had the chance to cross the bridge.
Residents also took the opportunity to walk along the initial stretch of the bridge while small boats with Croatian flags sailed beneath the six pylons.
Chinese Premier Li Keqiang was due to deliver a video message at the ceremony. His Croatian counterpart Andrej Plenkovic declared "tonight, we are uniting Croatia!" and described the bridge as a necessity, not a luxury.