The 22-year-old was recently released by Hull, having signed a three-year deal with the Tigers back in 2014.
He previously scored 26 goals in the 2013-14 season for non-league side Folkestone Invicta.
Ter Horst, recovering from a shoulder operation, told Maidstone's website: "This is a club definitely on the way up and I hope I can be a part of that."
National League club Maidstone United have signed former Hull City striker Johan ter Horst on a free transfer.