The pair outgunned Japan's 10th-seeded Shuko Aoyama and Ena Shibahara 6-4, 6-3 to post their 24th straight win at a major after triumphing last year at the Australian Open, Wimbledon and US Open.
"A big thanks to my partner Barbora," said Siniakova. "I'm so happy we did it again, it was a nice journey. Thank you for playing with me."
It was their seventh Slam title to make them only the fifth women's team in the Open-era to claim so many. Martina Navratilova and Pam Shriver hold the record with 20.
They broke Shibahara's opening service game to immediately put the Japanese pair on the back foot and never let up, repeating the feat in the second set to cruise home in 1hr 29mins.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)