Davies has joined the MLS club on a one-year loan deal, but United have the option to make the move permanent at the end of the season.
The 24-year-old has been training with United for the past two weeks.
“He simply looks terrific,” United General Manager Dave Kasper said. “Coming in, we weren’t sure where he would be, but he’s great physically, has impressed with his mental approach and we’re excited to see him on the field with us this season.”
“This past week in Florida has shown me how great the atmosphere around the team is, and I know this is a place where I can be successful,” Davies said.
“This is a group of guys that is very close, and I’m excited to be a part of it. Ben [Olsen] is a coach that will push me and get the most out of me, and the ultimate goal is to help this team turn it around for this upcoming season and to put D.C. United on the map.”