He became the first American to enter and be selected in an NBA draft directly after graduating from high school since the league first implemented age restriction rules in 2005.

[11] In March 2018 he confirmed his decision,[12] and later removed his name from the 2018 Nike Hoop Summit and Jordan Brand Classic.

[citation needed] Sources: Simons was selected 24th overall by the Portland Trail Blazers in the 2018 NBA draft.

[15] He would be the third high school player since 2015 to be drafted in the NBA, behind Thon Maker and Satnam Singh Bhamara.

[17] On April 10, 2019, Simons had his first career start against the Sacramento Kings while the Trail Blazers rested Damian Lillard and CJ McCollum for the final game of the regular season.

[25] On October 25, 2023, Simons suffered a torn UCL in his right thumb against the Los Angeles Clippers, ruling him out for four to six weeks.