The five foot snake managed to creep into the house in Dartford, Kent, after possibly using a hole in an outer wall.
Leanne Smith, 18, said: "We had been out for the evening and got home around 10.30pm and I wanted to bath Millie before I put her to bed.
Shocked: Leanne Smith with daughter Millie"I was sitting in the bath with her. I saw something move from the corner of my eye and then I heard this huge hiss!
"I looked round and there was this snake coiled up with its head up in the air as if it was going to attack.
"I screamed out for help to my partner Ben saying there was a snake in the bathroom."
Leanne's partner Ben Payne, 18, ran into the room and grabbed Millie and helped Leanne from the bath.
RSPCA officer Anthony Pulfer was called to the home to rescue the royal king python, which may have escaped from an owner in the neighbourhood.
A five-foot python (similar to pictured) slid into the bathroom as Leanne Smith bathed her four-month-old daughter MillieAlthough it is not a constricting snake or poisonous, the species can bite if it feels threatened, the RSPCA said. Both the woman and baby were unhurt.
Smith's neighbour, who has experience of looking after large snakes, has offered to care for the reptile while its owner is tracked down.
Mr Pulfer said: "This is a huge snake and it was quite a shock for the poor lady. Unfortunately, we have had to rescue lots of snakes lately.
"People take on these snakes and don't realise how big they can grow to and that they often escape."
An RSPCA spokeswoman said escaped pet snakes were often enticed into people's homes by the prospect of warmth.
Some can scale outer walls vertically to find a snug spot near hot pipes or central heating boilers.
She said: "Pythons can climb up the side of buildings. They will go anywhere it is warm so bathroom pipes are an obvious draw.
"We do get a lot of calls to rescue escaped snakes but to get one as large as this is a bit unusual."