The A487 is closed between the A4971 (Porthmadog) and A4085 High Street (Penrhyndeudraeth), according to reports from Traffic Wales and monitoring service Inrix.
It is expected to be closed “for some time”, and police have confirmed that the collision involved a fatality.
In a statement on Facebook North Wales Police said: “The A487 Porthmadog bypass remains closed due to a fatal collision. The road is expected to be closed in both directions for some time.
“Road users are advised to avoid the area and use the signed diversion route. Thank you for your patience whilst we deal with the incident.”
Chief Inspector Trystan Bevan of the Roads Policing Unit added: “I offer my deepest condolences to the families at this difficult time.
“This is sadly being investigated as a fatal road traffic collision.
“I am appealing to anyone who was travelling on the A487 between 21:00 and 21:40 on December 20 who witnessed the collision or may have dashcam footage, to get in touch with police.
“If you have any information, please contact us via webchat or by calling 101, quoting reference C196768.”
Traffic Wales also confirmed that the road is expected to remain closed on Sunday morning.
In a post on X, the service said: “Road remains closed due to a serious collision yesterday evening. Please avoid area and find alternative route.
“Road is likely to remain closed for some time.”