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No, Super Bowl LII was NOT Bill Belichick's final game in the NFL ... at least according to John Harbaugh who's convinced the Patriots coach is coming back.

With rumors swirling that Bill and Tom Brady could finally call it quits after the big game ... especially with 2 key assistant coaches likely leaving and Rob Gronkowski pondering retirement.

But Harbaugh -- the head coach of the Baltimore Ravens -- just ain't buyin' it ... telling TMZ Sports both Brady and Belichick are super tight and neither one is ready to live the civilian life just yet.

For the record, Belichick and Brady are adamant they're coming back next year.