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Henry III "the Great" (? – 1246) was the count of Sayn (1202–1246), a county located near the Sieg River in northern Rhineland-Palatinate, Germany. Henry III shared the first year of his reign with his uncle, count Henry II, as he and his father Eberhard II had co-ruled the county. Gottfried II (Count of Sponheim) had been a regent from 1181 and continued until his death in 1220. John, count of Sponheim-Starkenburg, was regent from 1226 until Henry's death in 1246.

In 1233, Conrad of Marburg, Conrad Dorso and John the One-Eyed accused Henry of indulging in satanic orgies. Henry pleaded his case successfully to an assembly of bishops in Mainz and was acquitted.^[1] Conrad, however, refused to accept the verdict, but eventually left Mainz. While passing near Marburg, Conrad was ambushed and killed by a group of knights. It is unknown whether they were in the service of Henry.

Henry died in 1246. His county was inherited by his brother-in-law, count Eberhard III of Sponheim-Eberstein.