A novel hybrid maize variety designated X5T123 and seed, plants and plant parts thereof, produced by crossing Pioneer Hi-Bred International, Inc. proprietary inbred maize varieties. Methods for producing a maize plant that comprises crossing hybrid maize variety X5T123 with another maize plant. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into X5T123 through backcross conversion and/or transformation, and to the maize seed, plant and plant part produced thereby. This invention relates to the maize hybridy variety X5T123, the hybrid seed, the hybrid plant produced from the seed, and variants, mutants, and minor modifications of hybrid maize variety X5T123. This invention further relates to methods for producing maize varieties derived from hybrid maize variety X5T123 and to the maize varieties derived by the use of those methods.