Footage of the brawl was captured on video and took place at the Championship clash on Saturday afternoon.
It has led to police officers and club officials warning people involved in the violence would receive "appropriate punishment".
Now West Midlands Ambulance Service has confirmed they responded to the incident and treated the five people at the scene.
A spokeswoman said: "Following a disorder at The Hawthorns on Saturday, we assessed and treated five patients. All had sustained minor injuries and were discharged on scene."
The people involved have been warned they could face lifetime match bans after fighting broke out, bottles were thrown and a gate at the stadium ripped open.
One of the brawls was captured on a video shared on social media and shows punches and kicks being thrown as a blue gate separating home and away fans is torn down.
Others can be seen making a break for it to escape the ugly scenes as a crowd storms the fence. Bottles and cans are thrown in both directions and among the violence is a man lying on the floor, apparently unconscious, as drinks land around him.
Some Baggies fans stood around the man in an attempt to protect him from the violence as others shout "We are Albion". Stewards can be seen in the video but no police are present.
Chief Inspector Ian Gorman, from West Midlands Police, said the force was now working with the club to track down those responsible. So far no arrests have been made and enquiries are ongoing.
He said: "We’re sad to say that there were pockets of fighting at the match on Saturday. Video of some of these fights was uploaded onto Facebook and Twitter which we’re using along with our own images and the club’s CCTV to find those responsible.
"People found guilty of crimes at fixtures face lifetime match bans as well as the sentences handed down by the courts. Anyone who can help our investigation into violence at the West Brom v Millwall game should contact us via Live Chat on our website west-midlands.police.uk."
West Bromwich Albion confirmed they were working with police and Millwall Football Club as they pledged anyone responsible would receive "appropriate punishment".
A spokesman for the club said: "We are aware of an incident which took place during the interval of yesterday’s fixture against Millwall, and a full review has begun in conjunction with West Midlands Police.
"We will work with police and Millwall Football Club to ensure anyone responsible for the disorder receives the appropriate punishment."