A novel soybean variety, designated XB15C11 is provided. Also provided are the seeds of soybean variety XB15C11, cells from soybean variety XB15C11, plants of soybean XB15C11, and plant parts of soybean variety XB15C11. Methods provided include producing a soybean plant by crossing soybean variety XB15C11 with another soybean plant, methods for introgressing a transgenic trait, a mutant trait, and/or a native trait into soybean variety XB15C11, methods for producing other soybean varieties or plant parts derived from soybean variety XB15C11, and methods of characterizing soybean variety XB15C11. Soybean seed, cells, plants, germplasm, breeding lines, varieties, and plant parts produced by these methods and/or derived from soybean variety XB15C11 are further provided.