The winger, 24, joined the Championship club on loan earlier in the season and has made five appearances for Neil Warnock's men so far, while also featuring heavily for their Under-21 side.
The Whites have now announced Hall will pen a two-and-a-half-year deal for an undisclosed fee in the new year, becoming the first permanent signing under the club's new owners GFH Capital.
Hall scored 14 goals in 54 appearances for Southend last season.
Southend's official website read: "Everyone at Southend United would like to wish Ryan all the best at his new club."
Meanwhile, Leeds boss Warnock told their club website: "When we brought Ryan in I said 'I believe he is a player with potential for a big future'.
"I'm sure during the length of his contract Ryan will have the opportunity to establish himself at this level, and you can see that from his contribution at Huddersfield a few weeks ago."
