Teenager Nathan French, of Merseyside, raised over £1000 for a dementia charity during his climb, but had to be treated for hypothermia by paramedics after only wearing a pair of Superman briefs.
The 1,085m Mount Snowdon peak is the highest in England and Wales, and Mr French told his local BBC affiliate that he was shaking uncontrollably due to the cold.
He said: “I was taken by surprise by how cold I got ... on the train down I started to feel really sick and I started going deaf and my eyesight started going funny.
“I started getting really emotional so my dad, who was with me, decided to call an ambulance.
“The paramedic who met me checked me over and said I was showing early signs of hypothermia.”
Phil Benbow, of North Wales Mountain Rescue Association, told the BBC that while undertaking climbs for charity was ‘laudable’, people should ensure that they are appropriately dressed.
Mr Benbow said: “People make an assumption that because there’s a cafe at the top and a train, that it’s a walk in the park. It’s not - it is the highest mountain in England and Wales and people need to respect that.
“Our team has been called to 173 rescues so far this year and that will be over 200 by the end of the year - the third year in a row we will have topped 200.
“That’s way too many for our volunteers who have to give up time with their family or at work. People just need to think ahead.”