Fire crews from across the city were called to HotSales Ltd, Showell Road in Bushbury, after reports of a factory blaze.
It happened at 7.26am today, West Midlands Fire Service confirmed.
A 33-year-old woman was arrested on suspicion of arson after being found on the site, said West Midlands Police.
Enquiries are ongoing in conjunction with the fire service to establish the cause of the fire, the force confirmed.
A total of 17 firefighters from four fire engines tackled the blaze, which was reduced to 14 firefighters an hour later.
A spokesman for West Midlands Fire Service said: "It's a severe fire in a single story factory unit which was 100 per cent involved in the fire.
"This fire is being treated as arson.
"Fire investigation is carrying out enquiries."