Officers want to speak to anyone who was travelling on the A469 between Ystrad Mynach and Caerphilly at around 6.20pm on Friday 15 December.
We received a report of a road traffic collision on the A469 near Llanbradach at around 6.20pm on Friday 15 December.
Officers attended, along with personnel from Welsh Ambulance Service, and the collision involved one car; a white Mazda.
The passenger of the car, a 20-year-old man from Pontllanfraith, was pronounced dead at the scene by paramedics.
His next of kin have been notified and will receive support from specialist officers.
The driver of the car, an 18-year-old man from Pontllanfraith, has been arrested on suspicion of causing death by dangerous driving and driving while under the influence of drugs. He remains in police custody at this time.
Officers investigating the collision want to speak to any motorists who were travelling on the A469 between 6pm and 6.30pm on Friday 15 December.
Anyone with information, including dashcam or CCTV footage, is asked to contact us by calling 101 and quoting log reference 2300426121.
Alternatively you can send us a direct message on social media or call Crimestoppers anonymously on 0800 555 111.