Wayne Gretzky's Edmonton Oilers jersey from the 1988 Stanley Cup-clinching game -- the Great One's last championship -- just hit the auction block ... and it could be the most expensive piece of hockey memorabilia ever sold.
TMZ Sports has learned ... the #99 jersey Gretzky rocked during the title-deciding game against the Boston Bruins is currently on the block at Grey Flannel Auctions ... and already has a bid over $400K!!
The craziest part ... there are still 18+ days left in the auction, and bidding doesn't typically heat up until the final day.