The 26-year-old Londoner has refused to sign a new deal and revealed he is eager for a new challenge after playing a key role in the Swans' promotion to the Barclays Premier League.
He told the club's official website, www.swanseacity.net: "I'd like to thank the chairman (Huw Jenkins), manager and especially the fans for a wonderful five years. But after five years I feel the time is right for a fresh challenge."
Pratley added: "It's been a difficult decision for me and my family to make because Swansea is a fantastic club with such friendly and passionate supporters.
"It's not going to be easy for me, but if there was a good time to leave then it's now, with the club having secured their place in the Premier League."
Pratley made 179 appearances for the Welsh club and scored 27 goals, including a stunning strike in the npower Championship play-off semi-final second leg win over Nottingham Forest.
Source: PA
Source: PA