The 23-year-old scored one goal in 17 appearances in all competitions after joining the League One club from Bristol City last summer.
The Gunjur-born professional player has previously had loan spells at Torquay United, Newport County and Shrewsbury Town.
Janneh becomes Wimbledon boss Johnnie Jackson's fourth signing of the January transfer window.
"Saikou is an exciting forward player who can play on both wings and through the centre," Jackson told the club website.
"He is different from what we have got already because he brings an electric pace."
Meanwhile, Saikou Janneh moved to England at the age of 13, starting his football career with Bath City before joining Bristol City FC.
