United officials said they did not want to comment.
The Odenton native attended the D.C. academy with three other teenagers who, in the past two years, signed homegrown deals: attackers Griffin Yow and Kevin Paredes and central midfielder Moses Nyeman. Those three received first-team minutes this year.
Greene, who played three times for the U.S. under-16 national team, has been in United’s youth system since 2015. He is a fullback who can play on either side, adding depth to positions that were thinned by injuries this year.
Joseph Mora, on the left, and Chris Odoi-Atsem and Oniel Fisher, on the right, were the only natural players in those spots. Russell Canouse and other midfielders filled back-line voids several times.
Greene committed to Maryland last year, but as his stock rose with United, his likelihood of playing NCAA soccer faded. The Terrapins are expected to announce their recruiting class in the next two weeks.
Greene will become the 15th homegrown signing in D.C. history, dating from 2009, when goalkeeper Bill Hamid joined the first team.
United has also been keeping a close eye on Jeremy Garay, a 17-year-old academy midfielder from Woodbridge, who is planning to attend the University of Virginia next year. He played for Loudoun United the past two seasons.