Jackson, who played for the University of Kansas for one season, is expected to be a top pick during tonight's NBA draft.
"I am thrilled to officially be a part of team Under Armour as I start my professional career," Jackson said in an announcement Thursday, calling the brand "a leading force in basketball today."
The Baltimore-based sports brand is the official apparel supplier of the NBA Combine and sponsor of the Jr. NBA.
Jackson was named Big 12 Freshman of the Year and second team All-American after a high school career in which he was ranked as the number one overall recruit by collegiate ratings programs.
His connection to Under Armour goes back to 2015, when he was named nationwide UAA Player of the Year.
"Josh brings a level of competitiveness and toughness that we’ve witnessed over the years at our Elite 24 and UAA grass-roots events," said Kris Stone, Under Armour's senior director of global sports marketing for basketball. "At every level of play, Josh has strived to win."
Under Armour's roster of basketball athletes also includes two-time NBA MVP and NBA champion Stephen Curry.