An arch support orthosis includes an arch curve being variably tensioned for supporting a user's arch and foot. The arch support orthosis includes a metatarsal curve, a curved heel portion and a continuous medial longitudinal arch curve. An anterior slope and posterior slope of the arch curve include varying thicknesses from the base of each slope to a crown of the arch curve. A greater base thickness of each anterior and posterior slope provides firm support of the respective anterior curve and posterior curve of a user's arch, with the crown being resiliently supportive of the user's arch during weighted and unweighted use. A means for tensioning having an adjusting means are connectable under the continuous arch curve between the anterior and posterior slopes, thereby allowing a user to adjust angles of the respective slopes and the tension along the arch curve for treating plantar fasciitis and other foot disorders.