The win gave Siniakova her 12th Grand Slam doubles title and made her the first player to complete the career Grand Slam with two different players having previously done it with fellow Czech Barbora Krejcikova.
After a sluggish start, Siniakova and Townsend came alive in the second set where the 2025 finalists fell behind 40-30 in the opening game before reeling off 23 of the next 27 points to force a decider.
The 2025 finalists then consolidated an early break with a Siniakova hold at love to move ahead 3-1 before Townsend served up a four-point game of her own for a 4-2 lead that set them on their way.
In the opening frame, the top-seeded pairing reeled off 13 consecutive points en route to reaching 4-4 before Siniakova, serving with the sun in her face and looking to force a tiebreak, double-faulted three times, including on set point.
Siniakova and Townsend won the Wimbledon women's doubles title in 2024, the Australian Open in 2025 and this year's French Open.