Southend twice came from behind at the Liberty Stadium to draw 2-2 and secure promotion to the Championship.
Freddie Eastwood scored both Southend's equalisers
Freddy Eastwood's brace cancelled out two goals from Swansea's Rory Fallon for a draw that proved enough after Brentford were held by Hartlepool.
Fallon blasted his first right-footed and headed his second from Alan Tate's cross to keep Swansea in the play-offs.
But each time Eastwood replied, first sliding home Mark Gower's pass then turning Kevin Austin to hit his second.
Swansea: Gueret, Ricketts, Austin, Tate, Monk, O'Leary, McLeod (Williams 82), Robinson, MacDonald, Trundle (Knight 71), Fallon.
Subs Not Used: Akinfenwa, Tudur-Jones, Forbes.
Goals: Fallon 19, 34.
Southend: Flahavan, Jupp, Wilson, Hunt, Barrett, Gower, Maher, Guttridge, Bradbury (Bentley 67), Goater (Gray 86), Eastwood.
Subs Not Used: Pettefer, Cole, Lawson.
Booked: Guttridge, Gower, Maher.
Goals: Eastwood 28, 63.
Ref: C Penton (Sussex).