County of Hainaut
Floris was supported by the count of [[County of Hainaut|Hainaut]] of the [[house of Avesnes]], who was an arch-enemy of the [[count of Flanders]] of the [[house of Dampierre]]. Floris married Beatrix of Dampierre, the daughter of [[Guy of Dampierre]], count of Flanders, in 1269.