| Călmățui | |
| Map: | Raul Calmatui.png |
| Subdivision Type1: | Country |
| Subdivision Name1: | Romania |
| Subdivision Type2: | Counties |
| Subdivision Name2: | Buzău, Brăila |
| Subdivision Type3: | Towns |
| Subdivision Name3: | Smeeni, Însurăței |
| Length: | 152km (94miles) |
| Source1: | South of Buzău |
| Mouth: | Danube |
| Mouth Location: | Gura Călmățui |
| Mouth Coordinates: | 44.8311°N 27.8272°W |
| Basin Size: | 1668km2 |
| Tributaries Left: | Rușavăț, Negreasca, Strâmbul, Buzoel |
The Călmățui is a left tributary of the Danube in Romania. Its source is a few kilometers southwest of Buzău. The 152km (94miles) long Călmățui flows generally eastwards and into the Danube at Gura Călmățui.[1] Its basin size is 1668km2.