The police and coastguard are carrying out inspections along the River Wyre after reports of a woman being seen on the wrong side of the railings on Shard Bridge in Hambleton.
Emergency services were called at 3.45pm with the police helicopter also deployed.
A Lancashire Police spokesman said: “A member of the public called to say that they had spotted a woman on the wrong side of the railings on the bridge.
“The coastaguard and a police helicopter were deployed to try track her down.
“Teams are still currently carrying out inspections of the river but no one has been found so far.”