It was the first major title for Russia's Kudermetova, while Belgian Mertens won her fifth Grand Slam in the category and a second Wimbledon triumph after her 2021 win with former partner Hsieh.
"It's been incredible," a tearful Mertens said as she summed up a memorable Wimbledon campaign, having also reached the fourth round in the singles.
Kudermetova held to go 3-1 up in the first set, but the fourth seeds then won five straight games. Kudermetova led the charge in the second set, holding twice as the pair got three breaks in a row to force a third set.
Hsieh, who has won seven majors in women's doubles including four Wimbledon titles, won a prolonged exchange with Kudermetova from the baseline to get the first break of the third set before Ostapenko held without losing a point to go 4-2 up.
But erroneous returns from Hsieh hit the net as the eighth seeds broke back, and Kudermetova held to go 5-4 up before Ostapenko went long to lose a 26-shot rally and bring up match point.
Kudermetova's backhand from close range cut across the court to secure the match, finally fulfilling her dream of lifting the trophy at the All England Club.
"A few years ago I was playing in the final here against Elise... I lost that final and it was so painful. Today I said to myself, I really want this," Kudermetova said.