Gerlach I of Isenburg-Arnfels was the Count of Isenburg-Arnfels from 1286 (1287) until 1303. Gerlach was the youngest son of Count Henry II of Isenburg-Grenzau. In 1286 he partitioned his lands between his sons, and Gerlach as the youngest received those around Bad Hönningen. He was succeeded by his son Theodoric in 1303.