Nelson declared for the draft last month, but did not sign with an agent, which allowed him to reverse his decision as long as he notified the NBA by Thursday.
"After considering all the options, the best decision for my family, my education, and my life is to return to school and complete my college career," Nelson said. "I love St. Joe's, the coaches, and my teammates, and the chance to help our program reach even greater heights is a once-in-a-lifetime thing. This is also a chance to give back to something that has given so much to me."
The 6-0, 190-pounder averaged 19.7 points and 4.7 assists per game as a junior. The two-time first-team all-conference choice in the Atlantic 10 is second in school history with 543 assists and 17th with 1,435 points.