The road was temporarily closed following a report of concern for a man at around 9.45am.
Emergency services, including the Scottish Ambulance Service, rushed to the scene between Junction 4 and Juntion 5 of the busy motorway.
It’s understood a 64-year-old man has been taken to hospital with serious injuries
Drivers are being warned to prepare for delays as congestion may take time to clear.
In a tweet, Traffic Scotland said: “The #M90 has been confirmed as REOPENED in both directions J4 - J5 following a police incident. Congestion may take a while to clear.”
A Police Scotland spokeswoman said: “Police in Fife were called to Junction 4 of the M90 following a report of concern for a 64-year-old man.
“The incident happened around 9.45am on Thursday, January 10. The Scottish Ambulance Service also attended and the man was taken to hospital.
“The M90 was temporarily closed in both directions but has now reopened. Members of the public are thanked for their patience at this time.
“Anyone with information regarding this incident is asked to contact officers on 101, quoting incident number 772 of January 10.”
Join our Facebook group Our Edinburgh to share images and news from and around the Capital