Abstract: A pivotable seat structure for a bicycle includes a seat post having a block formed on the top thereof, a positioning post passing through the rear end of the block, an abutting flange formed on the block, and an elastic pad mounted on the middle of the block, an inverted U-shaped supporting bracket defining a receiving space for receiving the block and having a front end pivotally mounted on the front end of the block and a rear end defining an sector slot, an inverted V-shaped driver having a front end formed with a hook detachably urged on the positioning post and a rear end passing through the sector slot and formed with a press block, a torsion spring having a front end abutting the front end of the driver for forcing the hook to urge on the positioning post, and a rear end urged on the bottom of the supporting bracket, and a seat fixedly mounted on the supporting bracket.