| Grenff | |
| Mouth Location: | Schwalm |
| Mouth Coordinates: | 50.8847°N 9.2569°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | Hesse |
| Length Km: | 21.9 |
| Basin Size Km2: | 86.4 |
The Grenff, also Grenf, is a 22 km long, right-bank or southeasterly tributary of the Schwalm. It passes through the East Hesse Highlands in North Hesse Schwalm-Eder-Kreis, and belongs to the river system and catchment area of the Weser.
The Grenff rises in Ottrauer Highland, a part of the Fulda-Haune Plateau in East Hesse Highlands, in the transition area to the Knüllgebirge.The source is 1.2 km southeast of the village Görzhain (municipality Ottrau) in a clearing of the northern flank of the Frohnkreuzkopf (about), the western spur of the Rimberg (591.8 m), at about 395 m elevation.
The Grenff initially flows north-west, to and through Görzhain. From there on the Bad Hersfeld–Treysa railway (also Knüllwald railway) runs through the Grenff valley. It passes along the settlement Bahnhof Ottrau and a number of watermills (Boßmühle, Steinmühle, Lenzenmühle, Schneidmühle) and the village Kleinropperhausen.Then the Grenff flows through the riverside town Nausis, past the Bruchmühle, Neukirchen, and Rückershausen, after which it passes the Sängermühle, and Riebelsdorf.Then it passes the station Zella and the Klinkemühle. At the village of Loshausen(Willingshausen municipality) it flows into the Eder tributary Schwalm that comes from the south, after descending 181 m in elevation.
The Grenff has a 86.406 km2 drainage basin. Its tributaries are orographic allocation (l. = on the left side, r. = on the right side) , river length and confluence location, Grenff river kilometer (starting from the mouth ):