Scannell, 25, spent two-and-a-half years with the Shrimpers following his move from non-league Eastleigh.
Daggers boss John Still told the club's website: "He is a powerful, quick and strong wide player, who can also play in the middle."
He added: "He is an exciting prospect and he has played at this level before.
"He is desperate to do well and hopefully we can help him fulfil his potential."
Scannell made 26 appearances for Southend as they were relegated from League One last season.
Shrimpers Captain Adam Barrett and Alan McCormack had their contracts cancelled earlier this week, with Simon Francis and Steve Mildenhall also looking for moves.
Physiotherapist John Stannard is leaving after not being offered a new deal.
Source: BBC Sport
Source: BBC Sport