Thomas had an assist in the clinching game, which sent the Blues to their first Stanley Cup Final since 1970.
Thomas has been staying at the home of Brady's dad, Keith Tkachuk, who played 543 games with the Blues during his 18-seasons in the NHL.
Brady Tkachuk wore a T-shirt representing Thomas' junior team, the London Knights of the Ontario Hockey League. The front of the shirt commemorated the Knights' Memorial Cup championship from 2016 while the back said "Thomas, Mr. STL" with his No. 18 on the back.
"He's the king of the city," Tkachuk told NBCSN. "He's been playing so well the fans love him."
Brady was last seen
rooting on his brother Matthew in the Calgary Flames
Western Conference First Round series defeat to the Colorado Avalanche. With their seasons over, the Tkachuk boys are home and the family has grown by one.
"He's part of the family," Brady Tkachuk said. "He's just another Tkachuk there."