威悉河（Weser）是所有完全位于德国境内河流中第二长的，仅次于美因河。其余更长的河流，如多瑙河、莱茵河、易北河、奥德河和摩泽尔河都有源头或者入海口在德国之外。

威悉河由富尔达河（Fulda）与威拉河（Werra）在汉明登附近汇合而成，流经黑森、北威、下萨克森和不来梅州，注入北海赫尔果兰湾（Helgolander Bucht）。依「河源唯远」的原则，发源于黑森州的富尔达河较短，故以较长的威拉河作为正源，因此威悉河还流经图林根州。452km长的威悉河加上威拉河共744km。

The Weser (German pronunciation:[ˈveːzɐ]) is a river in Northwestern Germany. Formed at Hannoversch Münden by the confluence of the rivers Fulda and Werra, it flows through Lower Saxony, then reaching the Hanseatic-town Bremen (see: Hanseatic League), before emptying 50 kilometres (31 mi) further north at Bremerhaven into the North Sea. On the opposite (west) bank is the town of Nordenham at the foot of the Butjadingen Peninsula; thus, the mouth of the river is in Lower Saxony. The Weser has an overall length of 452 kilometres (281 mi). Together with its Werra tributary, which originates in Thuringia, its length is 744 kilometres (462 mi).