North Wales Police said the last sighting of the man, identified only as John, was on Yr Wyddfa (also known as Snowdon) at 5.40pm on Monday.
The 20-year-old, who is described as around 6ft tall, was last seen on the Llanberis path, near to the railway line, officers said.
At the time of his last sighting, he was wearing black tracksuit bottoms with “Le Franz” written on them, hiking boots and a grey puffer jacket.
Eryri National Park says the Llanberis path is 14.5km long and this route to the summit of Wales’s highest mountain takes around seven hours.
The park authority says on its website that the path is classified as a “hard and strenuous route” with a “good level of fitness” required.
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In a statement, the force asked anyone with information who think they may have seen John to get in touch at the earliest opportunity using reference number C076334.
