The section of Bermondsey Street between Tanner Street and Long Lane can now be used by southbound cyclists.
The contraflow is part of a Transport for London-funded package of measures to improve conditions for cyclists in the area covered by the Barclays Cycle Hire scheme.
"We've been asking for this for years, and at last common sense won the day," says Barry Mason, coordinator of Southwark Cyclists.
"This proves you don't need huge infrastructure for worthwhile projects. Sometimes paint will do. Gallons more everywhere please!"
The contraflow is also supported by the London Cycling Campaign which is based in Newham's Row.
However not all residents of the street are so impressed – since the idea of the contraflow was mooted in 2007 there have been some forthright opinions expressed on the SE1 forum.
As part of the same programme of 'complementary measures' for the cycle hire scheme, cycle contraflows are also planned for Druid Street (between Tanner Street and Gedling Place), Guy Street, Kipling Street, Silvester Street, Stoney Street (between Park Street and Southwark Street) and Swan Street.