The road was closed in both directions between Caerwys and Northop on Saturday afternoon after an arctic lorry travelling eastbound had collided with two vehicles in the westbound carriageway.
Four people were taken to hospital – one was airlifted to hospital in Stoke by the Air Ambulance and the other was taken to Stoke via Ambulance. Two were also taken to Ysbyty Glan Clwyd by ambulance.
A major incident was declared by police at the scene.
The lorry driver – a 58-year-old man was arrested on suspicion of causing serious injury by dangerous driving. He has since been released under investigation whilst enquiries continue.
The road remained closed in both directions for several hours whilst officers from the Forensic Collision Investigation Unit carried out their initial investigation. One lane in both directions of the A55 re-opened shortly before midnight and a 40 mph speed restriction was in place throughout all of Sunday.
Traffic Wales have confirmed this morning that the road has fully reopened in both directions as of 7.20am.
Following the collision, North Wales Police issued an appeal.
Sergeant Danielle Ashley of the Roads Crime Unit, said: “We’d like to thank all those who have already provided us with their details following yesterday’s collision, however we are urging anybody who may have witnessed the collision, or who may have dash cam footage just prior to the collision taking place, to contact us as soon as possible.
“I’d also like to thank motorists for their patience whilst we dealt with this incident.”
Anybody with information is asked to contact officers at the Serious Collision Investigation Unit via the website or by calling 101 quoting reference number 25000657103.