The fire broke out at the fast-food restaurant on Bloxwich Road, between Green Lane and Blakenall Lane.
Video footage shared on social media soon after 4pm on Wednesday showed flames taking over at the popular restaurant as black smoke poured into the sky.
It is not yet known how long the restaurant will remain closed for while the damage is assessed.
A wooden outbuilding attached to the side of the Burger King was engulfed with flames which had spread to the fascia of the main building.
Firefighters also had to deal with a carbon dioxide cylinder which was being stored in the outbuilding, leading to the road being closed for safety reasons.
Three fire engines and a brigade response vehicle responded, from Walsall, West Bromwich and Bloxwich fire stations.
A spokesman for West Midlands Fire Service said: "At 4.02pm on Wednesday we responded to a fire at Burger King on Bloxwich Road, Walsall.
"Three fire engines and a brigade response vehicle responded, from Walsall, West Bromwich and Bloxwich fire stations.
"On arrival the firefighters were faced with a fire in an wooden outbuilding attached to Burger King, which had spread to fascia of the main building.
"A carbon dioxide cylinder which was being stored in the outbuilding was also affected by the fire, causing it to gas off, was made safe by the firefighters.
"The fire was extinguished quickly by firefighters in BA using a main jet and hose reels.
"To ensure the safety of our crews, whilst extinguishing the fire and dealing with the cylinder, the road was closed with the support of the police."
Burger King has been contacted for comment.