A novel maize variety designated PH12W5 and seed, plants and plant parts thereof. Methods for producing a maize plant that comprise crossing maize variety PH12W5 with another maize plant. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into PH12W5 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 PH12W5 or a locus conversion of PH12W5 with another maize variety.