According to the invention, there is provided a novel soybean variety designated XB40U08. This invention thus relates to the seeds of soybean variety XB40U08, to the plants of soybean XB40U08 to plant parts of soybean variety XB40U08 and to methods for producing a soybean plant produced by crossing plants of the soybean variety XB40U08 with another soybean plant, using XB40U08 as either the male or the female parent.