Disclosed is the seed of a novel soybean cultivar, designated 5.8 i, a sample of which is deposited under ATCC Accession No. PTA-7274. Also disclosed are plants, or parts thereof, grown from the seed of the cultivar, plants having the morphological and physiological characteristics of the 5.8 i cultivar, and methods of using the plants or parts thereof in a soybean breeding program.