A source told MLB.com that Underwood received $1,050,000, which is above the pick's slot recommendation of $769,600. The Cubs have not confirmed that figure.
Underwood, who turns 18 a week from Friday, will report to Rookie Level Mesa.
Underwood was 6-1 with a 2.36 ERA for Pope High School in Marietta, Ga., this past season, striking out 75 and walking 34 in 50 1/3 innings. The 6-foot-2, 205-pound right-hander struck out 15 in 6 1/3 innings on March 9 against Lassiter.
The Cubs have signed each of their first 20 picks and 29 players overall. They completed a deal with the No. 6 overall pick of the Draft, outfielder Albert Almora, on Wednesday.