A four-month-old baby is fighting for life after he was slashed in the face with a knife at a Brisbane home and a family member has been taken into police custody.

Police said the infant’s mother woke to the sound of his cries just after 6.30am on Tuesday and found the baby in his cot, with a second family member in the room "in possession" of a large knife.

Police collect evidence at a home in The Gap, where a baby was assaulted on Tuesday morning. Credit:Jorge Branco

Regional Duty Officer Inspector Daniel Bragg said paramedics and officers were “visibly distressed” by the scene at the home, in The Gap, in Brisbane’s west.

He said the baby suffered "several slash marks" to the face and the family member had been "in possession" of the knife.