The Isenach rises in the northern Palatinate Forest, southwest of Carlsberg Hertlingshausen. Its source in the Diemerstein Forest on the southeast flank of a saddle between the peaks Krummes Eck, elevation , and Hohe Bühl, elevation , is marked with Ritterstein number 277, with the inscription Isenach source. The first of the river flow in a southeasterly direction. After the Isenach passes the Isenachweiher reservoir, it flows east through a valley it shares with Bundesstraße 37, Kaiserslautern-Bad Dürkheim.