Sixth-ranked Pegula eliminated the 24th-ranked Russian in 83 minutes to book an all-American final on Monday against Amanda Anisimova, who ousted US eighth seed Emma Navarro 6-3, 2-6, 6-2.
Pegula, trying for the first Canadian repeat since Martina Hingis in 1999-2000, is 2-0 against Anisimova, most recently winning in April at Charleston.
"She's probably one of the biggest, cleanest ball strikers I've ever played against, so when she's on it can be really tough," Pegula said of Anisimova.
"Tomorrow definitely is going to be a battle and I'm going to have to do my best to just make her play a lot of extra balls and try to be aggressive when I can, because when she's on she can hit a lot of winners and take the racquet out of your hand sometimes, so I'm going to have to be smart tomorrow."
Shnaider, a 20-year-old left-hander, had a break point in the eighth game but netted a forehand and Pegula held for a 5-3 lead.
She then broke on the longest rally of the match, 21 shots, when Shnaider netted a backhand to end matters.
Pegula moves into her 13th career WTA final, seeking her sixth title and second of the year after Berlin in June.
Shnaider, who ousted US top seed and reigning US Open champion Coco Gauff, was coming off a silver-medal effort in Paris Olympics doubles.
