Firefighters were called after a group of students decided to have an early morning barbecue.
The crew, from Southsea, received a call at 3am to say neighbours had spotted fire in the garden of a house in Sandringham Road, Southsea.
When they arrived the students who lived in the property explained they had used the mattress as a barbecue.
It took firefighters around 20 minutes to put the blaze out. No one was hurt.
Crew manager Jamie Wren said the students were given a talking to about the dangers of their actions.