Second seed Raonic served 15 aces on his way to a 6-4 7-5 win over American Donald Young in their semi-final in Washington.
Young could only win 12 return points and failed to create a break-point opportunity against the powerful Raonic serve.
Raonic is yet to drop a set at the ATP 500 tournament, which is played on hard courts.
"I did the most important things. I took care of my serve. I thought he played really well and I just put that pressure on him in the final games of the set. It paid off," he said.
"Obviously I want more. I think I have a much better understanding of what I need to bring out of myself, especially earlier in tournaments so I can do consistently well at each tournament and give myself the opportunity to get into the weekend stages."
The Wimbledon semi-finalist's appearance in the final is his first decider since October last year, when he was beaten at the Japan Open, as he bids for a sixth ATP title.
Awaiting him in the final is Pospisil, who upset French sixth seed Richard Gasquet 6-7 (5-7) 6-3 7-5.
Pospisil, who had earlier in the day completed the third set of his win over Santiago Giraldo in the quarter-finals, had to come from behind against Gasquet.
He found himself 3-0 down in the deciding set before breaking back and again in the 11th game on his way to his first ATP Tour final.