Natasha Evans, 17, found the python at her home in Hebburn, Tyne and Wear and, terrified, called her mum, Trudi, 45, at work.
Trudi told her local paper: "Tasha said she had gone in the bathroom and without really looking leant down to put the plug in the bath and saw the snake out of the corner of her eye and ran out of the bathroom.
"She wasn't sure if she'd actually seen it or imagined it, so she opened the door and looked back in and there it was.
"Of course I didn't believe her. Or I thought if anything it was a little grass snake or a worm."
But it turned out the python had been on the loose since July after it escaped from a neighbour's collection.
Trudi added: "The family who used to live next door bred snakes and Tasha went on Facebook to message them and ask if they were missing a snake.
"They said it had been missing since they moved house in July and just thought it was dead so didn't look for it.
"So this snake has been living in the empty house next door since July and must have got under the floorboards and worked its way over to our house. It's terrifying."
The royal python has now been removed from the house, but Trudi said she hasn't been in the bath since and is scared to even go into the bathroom.
She said: "I know it is the kind of thing that makes people laugh because it's so unusual, but it could have been really dangerous as well.
"I'm going to be straight on the phone to get someone to come round and pull all the floorboards up to check there aren't any others."
Royal pythons are also known as ball pythons and are a non-venomous snake that comes from Africa.