Roberta Vaituke spotted the snake when she popped in to use the toilet.
She ran to alert her parents and brother.
They plugged up the sink to stop the snake from escaping and waited for the RSPCA to remove it from their house in Portsmouth, Hampshire.
The snake is believed to be a pet called Sammy which escaped from a tank in a neighbour’s home six months ago.
Sammy’s owner Harry Sharpe, 20, thought his pet would have died of malnutrition since he disappeared six months ago.
He said: ‘I’m not sure how he has survived for so long without any food. He must have been eating mice down there.
‘He’s a friendly snake. I just want him back now.’