A novel soybean variety, designated XB45J12 is provided. Also provided are the seeds of soybean variety XB45J12, cells from soybean variety XB45J12, plants of soybean XB45J12, and plant parts of soybean variety XB45J12. Methods provided include producing a soybean plant by crossing soybean variety XB45J12 with another soybean plant, methods for introgressing a transgenic trait, a mutant trait, and/or a native trait into soybean variety XB45J12, methods for producing other soybean varieties or plant parts derived from soybean variety XB45J12, and methods of characterizing soybean variety XB45J12. Soybean seed, cells, plants, germplasm, breeding lines, varieties, and plant parts produced by these methods and/or derived from soybean variety XB45J12 are further provided.