But what happened and when did the incident take place?
Here’s what you need to know.
When did the incident take place?
The incident happened at a house in Pentwyn, Penyrheol, Caerphilly, on Monday (8 November) at about 4pm.
Paramedics from the Welsh Ambulance Service went to the scene where the boy was pronounced dead.
Gwent Police said the dog was destroyed by firearms officers and officers would remain in the area while an investigation is carried out.
The street is in the middle of a large housing estate and a few police vehicles remain parked at the end of the road on Monday night, with police presence as Gwent Police continue their investigation on Tuesday morning (9 November).
‘The dog has been a nuisance on the estate for a bit of a time’
A nearby resident told the PA news agency: "The dog has been a nuisance on the estate for a bit of a time.
"I took our son down to the local shop a few days ago, and he was down there then and he was lunging at my son.
"My son is three years old and I had to pick up my son just to move around the dog because of the size on him."
Chief Superintendent Mark Hobrough said: "Officers will be making further inquiries at this time and will remain at the scene as the investigation progresses.
"It is possible that you may see ongoing police activity in Caerphilly as part of this work."
He added: "If you have concerns or information then please do stop and talk with us."
Member of the Senedd (MS) for Caerphilly Hefin David said: "My thoughts tonight are with the families involved. I know the close-knit community will rally round and offer support."
South Wales East MS Natasha Asghar added: "My heart goes out to this little boy and his family in Caerphilly who must be utterly devastated after what’s happened today."
What can I do if I have information?
Anyone with information on the incident is asked to call police on 101 quoting log reference 2100 392 510.
You can also send police a direct message on Facebook or Twitter.
You can also contact Crimestoppers anonymously by calling 0800 555 111.