According to the invention, there is provided a novel soybean variety designated XB21S05. This invention thus relates to the seeds of soybean variety XB21S05, to the plants of soybean XB21S05 to plant parts of soybean variety XB21S05 and to methods for producing a soybean plant produced by crossing plants of the soybean variety XB21S05 with another soybean plant, using XB21S05 as either the male or the female parent.