In the summer of 2018, Simmons' brother Liam resigned his position as assistant coach at the University of California, Riverside to work with Simmons on improving his shooting game. The decision to hire his brother as his shooting coach was not without controversy or criticism. His sister Emily, a former member of the Washington State University women's rowing team who has worked for Klutch Sports since 2014, is married to former NFL player Michael Bush. His godfather, David Patrick, was an assistant coach at LSU and was a part of his sole season there and is currently the head coach at UC Riverside. A cousin of Simmons, who had attended the 2016 NBA draft, was killed in a hit-and-run on 25 June 2016 in New Jersey.