Three people had to be taken to hospital after the blaze broke out on the first floor of a terraced house on Goldsmith Street, on Sunday.
A spokesman for Nottinghamshire Fire and Rescue said: “A fire investigation has been concluded into the fire and it looks to have most probably been deliberate.
“The police are taking it further.”
Firefighters from Mansfield and Ashfield attended the incident at about 4.30pm on Sunday.
They used four breathing apparatus and two hose reel jets were used to tackle the fire, which was out by 5.45pm.
The three people were taken to King’s Mill Hospital for precautionary checks.
Police confirmed the investigation has been launched.
If you have any information that may help them, call 101.