A novel soybean cultivar, designated 921021, is disclosed. The invention relates to the seeds of soybean cultivar 921021, to the plants of soybean 921021 and to methods for producing a soybean plant produced by crossing the cultivar 921021 with itself or another soybean variety. The invention further relates to hybrid soybean seeds and plants produced by crossing the cultivar 921021 with another soybean cultivar.