The 27-year-old will meet world No.1 Aryna Sabalenka, who was made to work hard for a 7-5 6-4 win over Italy's Jasmine Paolini to reach her fourth final in five years.
Ostapenko, who upset five-times grand slam champion Iga Swiatek in the last eight to improve her record to 6-0 against the Pole, got off to a strong start with a break in the first game.
She was broken straight back but earned another one quickly to go 4-2 up.
Alexandrova, who beat Mirra Andreeva and Jessica Pegula at the tournament to take her tally to five top-10 wins during the season, struggled with her opponent's thundering groundstrokes.
"I feel amazing playing here," Ostapenko said. "So pumped up. Those kinds of matches, winning three sets, gives you confidence," she said.
"I was putting a lot of balls in. I am very happy with myself."
Sabalenka, a three-time losing finalist in Stuttgart between 2021-23, looked to be powering through the first set after taking a 3-0 lead, but Paolini fought back to cut the deficit before losing it 7-5.
The tables were turned in the second with the diminutive Italian punishing a sloppy Sabalenka, who was rushing to finish the points with her trademark power but seeing errors pile up, and earning a 3-0 lead.
However, she was unable to maintain the standard.
"I was just trying to stay aggressive, to put as much pressure on her," Sabalenka said.
In France Elina Svitolina won the Rouen Open defeating Serbia's Olga Danilovic 6-4 7-6 (10-8). It is the 30-year-old Ukrainian's first Tour-level title for nearly two years.
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