Needing a win by four or more points in Newcastle, the Rebels left it late to reach the qualifying final, where the Reds await.
The winless Force almost held off the Rebels, but Cabous Eloff scored the decisive try in the 79th minute before Matt Toomua converted at McDonald Jones Stadium.
It marks the first time the Rebels have reached the finals in Super Rugby.
The Rebels threatened to put Force away during the first half.
Tom Pincus and Reece Hodge scored tries after charge-downs, either side of Force responding through Henry Taefu.
Hodge's try, coming after a fine pass from Cameron Orr, came after two Toomua penalties as the Rebels took a 20-8 lead.
However, Andrew Ready crashed over either side of half-time for Force to bring it level.
Force scored 22 unanswered points, making the most of a man advantage as Brynard Stander went over after Pincus's yellow card.
Dane Haylett-Petty ended that run after a break from Marika Koroibete, before Force's Byron Ralston was shown a yellow card for taking Matt Philip out in the air.
Trailing 30-27, the Rebels needed a converted try in the final 20 minutes to advance to the finals.
Koroibete had a try ruled out for a forward pass in the 73rd minute before the Rebels' chances were boosted when Fergus Lee-Warner was shown a yellow.
Force defended desperately late on, but the Rebels finally broke through, Eloff going over before Toomua converted from right in front of the posts to seal a memorable win.