Czech Siniakova and American Townsend held off Taiwanese veteran Hsieh Su-wei and Latvian Jelena Ostapenko to win 6-2 6-7 (4-7) 6-3 in their Australia Day decider on Rod Laver Arena.
Already in elite company as a "Golden Slam" doubles winner, it was Siniakova's third crown from four Australian Open women's doubles finals appearances, having won in 2022 and 2023 with compatriot Barbora Krejcikova.
Townsend claimed her first Australian Open title, and Siniakova her third among a major haul of 10. (James Ross/AAP PHOTOS)
It's her 10th major crown overall, while she has also won two Olympic gold medals in mixed and women's doubles.
It's Townsend's first senior victory at Melbourne Park, however in 2012 she won both the junior girls' singles and doubles titles as a 15-year-old.
"This means a lot to me," said the 28-year-old mother-of-one.
"The AO junior in 2012 was really the start of me believing it was even possible for me to play on a pro level.
"I won the doubles and then the singles and that was the start of me believing so this place means a lot to me andÂ
The top seeds seemed headed for a comfortable win after being up 5-3 in the second set, but a nervous Townsend faltered while serving for the match, with her fiesty partner tossing her racquet in disgust.
Katerina Siniakova was unhappy after they blew a chance to wrap up the title in straight sets. (James Ross/AAP PHOTOS)
That allowed their third-seeded rivals to level at 5-5, with the crafty Hsieh then holding serve for a 6-5 lead.
World No.1-ranked Siniakova continued her unblemished serving record to force the set to a tiebreak but she and Townsend couldn't stick with Hsieh and Ostapenko.
In the deciding set the teams went toe-to-toe until the eighth game when Hsieh lost her serve after some confusion on who would take the shot on the second break break point.
Siniakova then had chance to serve for the title and she made no mistake.
The Czech, who is world No.50 in singles, linked with Townsend after her Australian partner, former top-ranked Storm Hunter ruptured her Achilles in April, with the pair collecting their first major title at Wimbledon.
The win denied Hsieh, who is coached by former Australian Open tournament director and doubles legend Paul McNamee, an eighth grand slam doubles title with a fifth different partner.
Ostapenko, who won the French Open singles title in 2017, was bidding for her second in a row after teaming with Ukrainian Lyudmyla Kichenok to win the US Open last year.
Hsieh won the Australian Open last year with Belgian Elise Mertens, coincidentally beating Ostapenko and Kichenok.
Ostapenko, who is ranked 22nd in singles, only linked with Hsieh this year, with the pair playing their first tournament in Adelaide.
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