The Metropolitan Police said the body was found in the Grand Union Canal near Old Oak Lane.
Officers and paramedics were called just before 1.20pm on Sunday.
The baby was pronounced dead at the scene.
Cordons are in place at the canal and an urgent investigation is under way to establish the circumstances.
Police are urging the baby’s mother to seek medical assistance by dialling 999.
Anyone with information is asked to call 101 quoting reference 3589/09.
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