A novel maize variety designated PHHJN and seed, plants and plant parts thereof. Methods for producing a maize plant that comprise crossing maize variety PHHJN with another maize plant. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into PHHJN through backcross conversion and/or transformation, and to the maize seed, plant and plant part produced thereby. Hybrid maize seed, plant or plant part produced by crossing the variety PHHJN or a trait conversion of PHHJN with another maize variety. Inbred maize varieties derived from maize variety PHHJN, methods for producing other inbred maize varieties derived from maize variety PHHJN and the inbred maize varieties and their parts derived by the use of those methods.