Midfielder Jayson Molumby found himself in a heated altercation when he struck his opponent in the face.
The 24-year-old Ireland international reacted to a strong but legal challenge from Portuguese midfielder Samu Costa.
The two exchanged heated words before butting heads, with Costa pushing Molumby’s neck in response.
This enraged the West Brom player, who then delivered a powerful right hook and wrestled Costa to the ground, creating a shocking scene.
A brawl ensued as teammates from both sides rushed to separate them.
John Swift was the first West Brom player to intervene, reaching over the pair, followed by the referee and linesmen.
The incident occurred in the 40th minute, with the score still 0-0 at West Brom’s Walsall training ground in the West Midlands.
Both teams agreed to end the first half two minutes early to let tempers cool down, and neither player was sent off.
However, both Molumby and Costa were substituted during the break by their managers.
Fans from England and Spain were stunned as clips of the incident circulated on social media.
This was Molumby’s first start since January after undergoing foot surgery.
The incident came just days after Chelsea’s Levi Colwill and Wrexham’s James McClean clashed early in their pre-season 2-2 draw in America, although their confrontation was less severe and ended without major incident.