A novel maize variety designated X90K666 and seed, plants and plant parts thereof are produced by crossing inbred maize varieties. Methods for producing a maize plant by crossing hybrid maize variety X90K666 with another maize plant are disclosed. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into X90K666 through backcross conversion and/or transformation, and to the maize seed, plant and plant part produced thereby. This invention relates to the maize variety X90K666, the seed, the plant produced from the seed, and variants, mutants, and minor modifications of maize variety X90K666. This invention further relates to methods for producing maize varieties derived from maize variety X90K666.