Fultz made the announcement in a video on Twitter.
"After sitting down with my mom and the coaching staff, I've made the decision to enter the NBA draft," Fultz said in the video. "Although the season hasn't gone as planned, it's really truly been a blessing to be here, and I learned plenty of life lessons on and off the court, especially from coach [Lorenzo Romar] and his coaching staff. I'm always going to be a Husky for life. Go Dawgs."
Fultz, a 6-foot-4 guard from Maryland, averaged 23.2 points, 5.9 assists and 5.7 rebounds per game in his one season at Washington. He missed six of the final eight games of the season with a sore knee.
"Markelle has decided to enter the NBA Draft and we totally support his decision," Romar said in a statement. "We appreciate him giving his all, on and off the court, while he was part of our program and I will enjoy watching him play for many years to come."
Fultz is the No. 1 player on Chad Ford's Big Board heading into the NCAA tournament and has been considered the front-runner for the top pick since the opening weeks of the season.
The DeMatha Catholic High School product was ranked No. 7 in the ESPN 100 for the 2016 class and chose Washington over Arizona and Louisville. Fultz didn't play on the varsity team at powerhouse DeMatha until his junior year, but he rose to five-star status after winning Washington Catholic Athletic Conference Player of the Year as a junior.
"The last three years of Markelle Fultz's basketball career have been nothing short of amazing," Romar said. "He has made one of the quickest improvements of any athlete I've seen in a long time."
Washington finished 9-22 (2-16 Pac-12) in Fultz's one season in Seattle, but will bring in the No. 3-ranked recruiting class in the country next season.