Leicestershire Police said the officer was bitten on his upper leg when two officers attended an address in Ibstock on Thursday afternoon.
The officer was taken to hospital on Thursday night to be treated for his injuries, while the second officer suffered a small cut to his finger.
A spokesperson for Leicestershire Police told the BBC: “Officers attended an address in Lewis Close, Ibstock at around 4.40pm yesterday (26 October) with regards to a man wanted by police. On entering the address, two officers were attacked by a dog which was being kept in the kitchen.
“One officer sustained serious bite injuries to his upper leg and remains in hospital. The second officer sustained a small laceration to his finger.
“Although the dog has not been further examined, it is believed to be a XL Bully breed. This dog and a second dog, a pit bull type breed, were seized from the address and remain in secure kennels.
“Enquiries are ongoing into the incident. There have been no arrests.”
There have been a number of XL bully attacks in the last few months, increasing calls for the breed to be banned.
The Environment Secretary, Therese Coffey, said earlier this week that the government is “pretty close” to bringing forward its proposed ban on American XL bullies.
Therese Coffey said she thought a “good definition” of the dog type had been agreed, with only a “few other things” such as compensation needing to be signed-off.
This attack comes following new figures showing that nearly 3,500 banned dogs are still living at home with their owners, with thousands more unregistered on official records.
In the most recent reported attack last week, a 60-year-old woman in Brisley, Norfolk, was mauled by her own XL bully dog, which escaped and then had to be put down.