According to the invention, there is provided a novel soybean variety designated 91M91. This invention thus relates to the seeds of soybean variety 91M91, to the plants of soybean 91M91 to plant parts of soybean variety 91M91 and to methods for producing a soybean plant produced by crossing plants of the soybean variety 91M91 with another soybean plant, using 91M91 as either the male or the female parent.