The County of Dassel (German: Grafschaft Dassel) emerged shortly after the turn of the 11th and 12th centuries when, after the extinction of the male line of the Billungs, its seat in Suilbergau, north of the Solling hills was divided into the domains of Einbeck and Dassel. Reinold of Dassel was able to secure rights similar to comital rights. The county lasted about 200 years, till it was abandoned in 1310 when there were no heirs. The most prominent member of the comital family was Rainald of Dassel, chancellor to Emperor Frederick Barbarossa and Archbishop of Cologne.
The family strengthened its domain by building a castle at the family seat in Dassel.
At the beginning of the 13th century, there was a spirit of optimism all over the county, which allowed the economy and trade to flourish. The height of the county's prosperity was in the mid-13th century. The county was broken up by sales of property around the end of the 13th century. It disappeared finally in the early 14th century, with the absence of male descendants.
At the beginning of the 12th century, the county covered the forested region of the Upper Weser (roughly the area of the present-day Solling-Vogler Nature Park) and its eastern foreland as far as the Leine valley.
It was subsequently divided into two parts due to a split in the family tree. The Adolfic line, with its seat at Hunnesrück castle in the northern part of the county, fell briefly to the county of Ratzeburg at the beginning of the 13th century, its domain thereby increasing considerably. The county of Ratzeburg was lost again very soon thereafter, however, as a consequence of its losing the Battle of Waschow.