Dockers goalsneak Hayden Ballantyne and skipper Matthew Pavlich booted four goals apiece as the home side cruised to the 21.8 (134) to 9.6 (60) victory.
The loss was arguably Port Adelaide's worst effort of the year.
But they had plenty of reasons to celebrate after the game as news filtered through that Essendon had upset Carlton in Saturday night's game at the MCG.
That result guaranteed Port Adelaide would finish in the top-eight, even if they lose to the ninth-placed Blues at AAMI Stadium next Saturday.
The finals berth under new coach Ken Hinkley caps a remarkable turnaround for the Power, who won just five games last year in a campaign that cost Matthew Primus his job.
Fremantle leapfrogged Sydney into third spot on the table following their fifth straight win.
But the Dockers need Brisbane to upset Geelong at Simonds Stadium next week if they are to nab second spot.
Fremantle defender Nick Suban is in danger of missing the start of the finals after injuring his right hamstring in the opening term.
Ballantyne entered the match under his own injury cloud after damaging his right shoulder against Melbourne the week before.
The 26-year-old didn't appear all too bothered in the opening half, booting three goals and causing all sorts of problems at ground level as the Dockers opened up a match-winning 55-point lead.
But it wasn't all smooth sailing for the little maestro, who sent a scare through the Dockers' camp in the third quarter when he grabbed his shoulder in pain after being tackled hard to the ground.
Ballantyne immediately went off for treatment, but he was given the green light to resume and kicked his fourth goal shortly after.
Michael Walters and Ballantyne combined for the first four goals of the match as the Dockers' frenzied pressure took its toll on the Power.
Ballantyne's pressure at ground level was simply brilliant, with Power defenders often forced to rush their disposal as the Dockers goalsneak closed them down.
Fremantle slammed through five unanswered goals during the second term, but Ballantyne's third major was a highly controversial one.
Ballantyne should have had little trouble as he ran onto a loose ball in the goalsquare after Walters punched it into his path.
But a wicked bounce saw the ball evade Ballantyne's swinging boot, with the Docker hanging his head in disappointment as the ball dribbled over the goal line.
However, the goal umpire awarded the goal without going to a review after deeming Ballantyne had made contact.
FREMANTLE 5.4 10.4 15.7 21.8 (134)
PORT ADELAIDE 1.1 1.3 4.4 9.6 (60)
Goals: Fremantle - Ballantyne, Pavlich 4; Walters 3; Sheridan, Clarke 2; Sutcliffe, Hannath, Neale, Barlow, Johnson, Crowley. Port Adelaide - Monfries, Moore, Young, Ebert, Wingard, Neade, Schulz, Mitchell, Gray.
Best: Fremantle - Ballantyne, Neale, Fyfe, Crowley, Pavlich, Johnson, Walters. Port Adelaide - Cornes, Westhoff, Gray, Wingard.
Umpires: Jeff Dalgleish, Shane McInerney, Dean Margetts.
Official Crowd: 35,565 at Subiaco.