The closures are being made as a precaution while a controlled explosion takes place.
A police spokesman said: “This is following the discovery of materials believed to be used in the demolition process at the site of the new Queensferry Crossing.
“Although there is no immediate danger to the public, a decision has been made to dispose of these materials with the assistance of the Explosive Ordinance Disposal Unit and to close the road as a precaution during this time.”
Chief Inspector Irene Ralston said: “Exposed wires were discovered today at the Queensferry Crossing, and while this is poses no immediate risk to the public, as a precaution we are compelled to close the Forth Road Bridge and the M90.
“We would advise motorists to avoid the area between 4pm and 6pm and seek alternative routes.”