The child also suffered a fractured leg and a bruised bottom after her mother left her in his care for an hour, Geraint Walters, for the prosecution, told Swansea Crown Court.
The girl, who cannot be named for legal reasons, needed more than three weeks' treatment in the burns unit of St Lawrence Hospital, Chepstow, Gwent.
'For reasons best known to this defendant he lost his cool and deliberately placed her in the bath of extremely hot water,' Mr Walters said.
The 27-year-old stepfather denies child cruelty and causing grievous bodily harm with intent. Judge John Diehl made an order banning his identification.
The stepfather claimed he left the bathroom to give his baby a bottle in another room. When he returned she was lying on her back under the water.
The trial continues today.Reuse content