In 2015, Wisconsin traveled to Hawai'i and beat the Rainbow Warriors 28-0. Nelson was on the losing squad that day, and the Badgers were on his mind when he made the decision to transfer back to the mainland. Nelson was an honorable mention all-state receiver during his high school days in Maryland, and decided to head to the islands to attend college. He played in 10 games, starting eight, as a true freshman, posting 36 tackles and six pass breakups. Nelson started all 13 games in that 2015 season (53 tackles, 1.5 for loss, 15 pass breakups, forced fumble) before moving to Wisconsin to redshirt the following year as a transfer student. He met all expectations in his only year in the cardinal-and-white, starting all 14 games and finishing tied for third in the country with 21 passes defended (no interceptions). He was named first-team All-Big Ten after the regular season after being credited with 35 tackles, a blocked kick, and served as the team's punt returner (24 returns, 206 yards, one touchdown).