Dettifoss (Icelandic pronunciation: [ˈtɛhtɪˌfɔsː] ) is a waterfall in Vatnajökull National Park in Northeast Iceland, and is reputed to be the second most powerful waterfall in Europe after the Rhine Falls. Dettifoss is situated on the Jökulsá á Fjöllum river, which flows from the Vatnajökull glacier and collects water from a large area in Northeast Iceland. The sediment-rich runoff colours the water a greyish white.

The falls are 100 metres (330 ft) wide and have a drop of 44 metres (144 ft) down to the canyon Jökulsárgljúfur. It is the second largest waterfall in Iceland in terms of volume discharge (behind the Urriðafoss), having an average water flow of 193 m³/s. The superlative of "most powerful" comes from its water flow multiplied by its fall distance.