The 21-year-old Philippines left-hander delivered an impressive victory over the four-time Grand Slam winner from Japan to reach her second career WTA final, having dropped the first last year at Eastbourne.
Giant killer Eala, ranked 28th, had previously ousted second seed Elina Svitolina for her sixth top-10 victory of the year after dethroning 2025 Washington champion Leylah Fernandez.
Osaka, ranked 13th, was thwarted in her bid for an eighth career WTA title and first since taking the 2021 Australian Open.
Pegula, seeking her 12th career WTA title and third of the year, eliminated Russia's Diana Shnaider 7-5, 6-4 in the other semi-final.
Pegula, who won her first WTA title in 2019 at Washington, fell behind 5-2 at the start, but the 32-year-old American top seed battled back to reach her fourth final of the year.
"I found a little rhythm on the return, served a little bit smarter and I was able to play some good tennis, kind of take control of the momentum," Pegula said.
'Going to be great'
Pegula beat Eala in a 2025 Miami semi-final in their only career meeting.
"I'm more well rounded. I'm physically a little bit better. I've improved on a couple of my strokes. Serve has also improved quite a bit," Eala said of her game then and now.
After an early exchange of breaks in the first set, Osaka saved three break points in the ninth game but followed with two errant forehands to surrender a break and Eala, who won 79% of her second-serve points, held at love on a service winner to capture the set in 38 minutes.
Osaka double-faulted away a break to begin the second set and netted a backhand to surrender another break and hand Eala a 5-2 edge.
Eala then held serve to end matters after 76 minutes.
Pegula, a semi-finalist at this year's Australian Open, has lifted trophies at Dubai and Charleston this season.
In the men's semi-finals, world number 10 Taylor Fritz will face 33rd-ranked Brandon Nakashima in an all-American matchup while Chile's Alejandro Tabilo meets 19-year-old Spaniard Rafael Jodar.