The Irishman's four-year stay at Plymouth saw him find the net 25 times in 191 appearances.
Meanwhile Evans, who started his career as a Chelsea trainee, will be most notable to Swindon fans, where he played 125 games between 2003 and 2006.
More recently he has featured for Blackpool, Millwall, Bradford and finally Bristol Rovers.
Southend manager Paul Sturrock has been busy assembling a squad since being named as the League Two side's new manager on 5 July.
Nine other players have already signed pre-contract agreements with the Essex side, who face an administration hearing in the High Court on 2 August.
Goalkeeper Glenn Morris, defenders Moshni Bilel, Sean Clohessy and Peter Gilbert, midfielders Craig Easton, Ryan Hall and Sofiene Zaaboub and forwards Barry Corr and Blair Sturrock are the other players who have joined Southend during the past fortnight.
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