Joe Flacco Jersey Sales Up 400% Amid Browns' Improbable Win Streak

The Dawg Pound is rallying behind Joe Flacco with their wallets amid the Browns' surprising winning streak ... with fans rushing to buy the backup-backup-backup QB's jersey!!!

The former Super Bowl winner's journey back to the gridiron has been one of the biggest storylines of the season -- he went from sitting on the couch on Sundays to leading the Browns to a 4-1 record after Cleveland lost Deshaun Watson and rookie Dorian Thompson-Robinson to injuries.

TMZ Sports has learned ... No. 15 threads have been a hot commodity since Flacco took over under center in December.

Fanatics -- the league's official e-commerce partner -- tells us Flacco is among the top 20 most popular jerseys in the NFL ... and is currently the top-selling Browns player.

In fact, we're told the sales spiked more than 400% during the span of his first victory on Dec. 10 to his playoff berth-clinching performance on Thursday Night Football against his former New York Jets team on Dec. 28.

FYI, Flacco has led the Browns with 1,616 yards and 13 touchdowns in just five games ... which is more than he had when he played for the Jets in TWO seasons.

It's been quite the feel-good story after Flacco tried to find a new home for the past five seasons. Remember, Joe signed with the Denver Broncos, Jets and Philadelphia Eagles after spending 11 years with the Baltimore Ravens.

He won Super Bowl XLVII with the Ravens in 2013, their second in franchise history.