This was just Popov's fourth appearance at a major.
The winner's check of $675,000 is more than six times her entire career earnings before Sunday. And just three weeks ago, Popov was ranked No. 390 and pushing a trolley for her best friend, Anne van Dam, at the Drive On Championship in the LPGA's restart.
She didn't play like a massive outsider this week, especially on Sunday — the biggest day of her career by far.
Starting out with a three-stroke lead, Popov drove into a bunker on No. 1 and pushed a 10-foot par putt right. She barely made a mistake after that.
Popov pumped her fist after rolling in a birdie putt from 8 feet at the second hole, then made another from a similar distance at No. 3.
Suwannapura, who would also have been an improbable winner with a ranking of No. 138, made four straight birdies from No. 4 to move within one stroke of the lead but it was the closest she came.
Birdies by Popov at Nos. 15 and 16 were greeted with furious fist pumps and left her on the brink. She held her nerve on the final two holes, parring No. 17 and then playing No. 18 cautiously to leave herself three putts to be champion.
After rolling a par putt within a feet of the cup, she marked her ball and started to cry. Within a minute, she had become the first female player from Germany to win a major title.
It was the second straight upset win at the Women's British Open. Last year, 20-year-old Japanese player Hinako Shibuno triumphed when playing her first event outside Japan.
This was the first women's major of a pandemic-disrupted year. It was played without spectators at Troon because of coronavirus restrictions.