The 23-year-old former Stoke City, Alfreton Town, Nuneaton Town and Worcester City centre-half, also capable of playing at right-back or in a holding midfield role, has signed an initial one-year contract with the Pilgrims.
Reflecting on Keane's arrival, United boss Adam Murray said: "Jordan is the type of player we have been holding out for. He's a defender who we feel is competitive, dominant but can also play.
"He has been captain a number of times so he brings a leadership quality to the group.
"Jordan was chosen from the non-league elite to attend the Vardy Academy, which upped his standing in the game. He has had a lot of interest over the summer but he has chosen to join our project which is very pleasing from our perspective".