Keaton Wood has agreed a permanent deal at Dartford after impressing there on loan last season.
Wood was a Millwall player when Darts boss Tony Burman brought him to Princes Park on an initial one-month deal in March.
He stayed with the Darts until the end of the season and following his release by the League 1 club earlier this summer, decided to retrace his steps ahead of the 2016/17 campaign.
Burman said: "I am pleased that we have signed a good, young defender who can play anywhere across the back line.
"When Keaton was with us on loan at the end of last season I was very impressed with his attitude and desire to win.
"Potentially I think we have signed another good prospect for many years to come."
Wood is Dartford's second summer signing after fellow defender Tom Bonner rejoined the club from local rivals Ebbsfleet at the start of June.