Ducati rider Bautista pipped Yamaha man Razgatlioglu by just over a tenth of a second at the end of a breathless 20-lap encounter on Sunday afternoon, as the lead changed hands between the pair countless times.
Razgatlioglu led heading into the final lap and looked as if he had done enough to edge out Bautista for what would have been only his third win in a full-length race of the season.
But Bautista was able to close up to his opponent in the final sector of the track, and as Razgatlioglu slid exiting the penultimate corner, Bautista was able to find the grip to go all the way around the outside of the final right-hander and take the chequered flag by 0.136s.
It completed a clean sweep of wins at Portimao - bringing his total for the year up to 24 out of a possible 33 - and gives him an almost-unassailable championship lead of 60 points heading to this month's final round at Jerez.
Bautista needs to score just two points, the equivalent of a 14th-place finish, to wrap up a title that has looked like a virtual inevitability for most of the season, despite Razgatlioglu closing the gap in recent rounds.
The Spanish rider celebrated another milestone earlier on Sunday in the Superpole race, as he matched Carl Fogarty's record of 55 wins aboard a Ducati.
Ducati sealed the manufacturers' title with Bautista's victory in the opening race of the weekend on Saturday, while the Italian marque was also able to celebrate Nicolo Bulega clinching the World Supersport title with a round to spare.
Michael Ruben Rinaldi, whose seat next season will be taken by Bulega, completed the podium in the final race of the weekend.
Jonathan Rea was third for Kawasaki behind Bautista and Razgatlioglu in the opening race, but crashed out of the Superpole race and then got caught up in an incident with Scott Redding and Xavi Vierge at the start of the finale.
After picking up a long-lap penalty, Rea eventually finished 10th to cap off a difficult weekend.
Team-mate Alex Lowes meanwhile sat out the final race completely after a crash in the Superpole race aggravated his existing left knee injury.
