The 21-year-old from Bracknell pleaded guilty at Reading Crown Court to one count of GBH with intent and one count of ABH, following the discovery of a number of injuries on the child last September.
The child was found to have sustained a fractured skull as well as numerous bruises to his face and body.
The attacker - who cannot be named for legal reasons - is due to be sentenced at Reading Crown Court on February 25 and has been remanded in custody until that time.
Det Con Dale Purcell, from TVP child abuse investigation unit, said: "This case is an appalling example of the abuse of a vulnerable 13 month old baby.
"My team and I worked tirelessly to ensure that justice was sought and that this child is now safe.
"Our joint work with social services and healthcare professionals on this matter was second to none and everybody involved can be very satisfied with the outcome".