According to the invention, there is provided a novel soybean variety designated XB31K08. This invention thus relates to the seeds of soybean variety XB31K08, to the plants of soybean XB31K08 to plant parts of soybean variety XB31K08 and to methods for producing a soybean plant produced by crossing plants of the soybean variety XB31K08 with another soybean plant, using XB31K08 as either the male or the female parent.