Raonic saved two match points to beat in-form Frenchman Ugo Humbert 6-3 3-6 7-6 (9-7) in his match, and Schwartzman had to wait for Rafael Nadal's victory over Pablo Carreno Busta before being able to celebrate securing a first ATP Finals appearance.
Medvedev has now won all four of his matches against Schwartzman.
The world No.5, whose seven ATP titles have all come on hard courts, broke Schwartzman's serve in the third game and did not let up.
Schwartzman never looked like mounting a comeback, winning just one of 28 points on Medvedev's first serve.
"Milos (Raonic) is one of the biggest servers maybe in the history of sport," Medvedev said after the match.
"So it's not going to be that easy, but for sure when you play indoor hard courts you need to serve well.
"So if I manage to carry this on, then I have a big chance to either go for a tie-break or have more chances to break the guy."
The ATP Tour Finals, being played in London for the last time before moving to Turin, begin on 15 November.
Canadian Raonic hammered 25 aces in a typically strong serving display, but had to battle back from the brink of defeat in a deciding-set tie-break against Humbert.
The 22-year-old Humbert was on an eight-match winning streak after triumphing in his last tournament in Antwerp.
But Raonic saved a match point on his opponent's serve after a remarkable 30-shot rally, before some big serves saw him over the finish line.
The former Wimbledon runner-up, who lost the 2014 Paris Masters final to Novak Djokovic, has been beaten in both of his previous meetings with Medvedev.
"It's going to be a tough one, he's been playing especially well this week," Raonic said. "I've just got to be sharp and take care of my serve."