多瑙河（德文：Donau）是欧洲第二大河，次于俄罗斯的伏尔加河，也是欧洲极为重要的国际河道，全长2857公里，流域面积81.7万平方千米，平均流量为6500立方公尺/秒。发源于德国黑森林地区，流经德国、奥地利、斯洛伐克、匈牙利、克罗地亚、塞尔维亚、罗马尼亚、保加利亚、摩尔多瓦和乌克兰等10个中欧及东欧国家，是世界上流经国家最多的河流，最后从多瑙河三角洲注入黑海。多瑙河流域范围还包括波兰、瑞士、意大利、捷克、斯洛文尼亚、波斯尼亚和黑塞哥维那、蒙特内哥罗、马其顿共和国、阿尔巴尼亚等9个国家，有大小300多条支流。

The Danube (/ˈdænjuːb/ DAN-ewb, known as various names in other languages) is Europe's second-longest river, after the Volga River, and also the longest river in the European Union region. It is located in Central and Eastern Europe.

The Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries. Originating in Germany, the Danube flows southeast for 2,860 km (1,780 mi), passing through or touching the border of Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine before emptying into the Black Sea. Its drainage basin extends into nine more countries.