The pair added to their gold in the duet technical, receiving a score of 97.680 points at Barcelona's Palau Sant Jordi arena.
Jiang Tingting and Jiang Wenwen gave China the silver with 95.350, while Spain's Ona Carbonell and Margalida Crespi Jaume took bronze with 94.990.
Russia is now five-for-five in synchro and widely expected to repeat its sweep of all seven events at the 2011 worlds in Shanghai.
Romashina already has quite a haul, winning four golds in Barcelona to give her 14 for her world championship career.
Results from the swimming world championships:
Open Water Swimming 5K Team Event:
1. Germany (Christian Reichert, Thomas Lurz, Isabelle Harle), 52:54.9.
2. Greece (Spyridon Gianniotis, Antonios Fokaidis, Kalliopi Araouzou), 54:03.3.
3. Brazil (Allan Do Carmo, Samuel De Bona, Poliana Okimoto), 54:03.5.
4. Australia (Simon Huitenga, Jarrod Poort, Melissa Gorman), 54:16.1.
5. Italy (Simone Ercoli, Luca Ferretti, Rachele Bruni), 54:34.0.
6. United States (Andrew Gemmell, Sean Ryan, Haley Anderson), 54:44.7.
7. France (Damien Cattin-Vidal, Bertrand Venturi, Aurelie Muller), 55:26.3.
8. Russia (Evegenii Drattcev, Kirill Abrosimov, Elizaveta Gorshkova), 56:08.7.
9. Hungary (Mark Papp, Eva Risztov, Anna Olasz), 56:09.4.
10. New Zealand (Kane Radford, Cara Baker, Phillip Ryan), 56:12.0.
Women's 10m Platform diving Final:1. Si Yajie, China, 392.15 points.
2. Chen Ruolin, China, 388.70.
3. Iuliia Prokopchuk, Ukraine, 358.40.
4. Sarah Barrow, Britain, 346.45.
5. Maria Kurjo, Germany, 336.55.
6. Pandelela Rinong Pamg, Malaysia, 334.55.
7. Maria Betancourt, Venezuela, 328.35.
8. Roseline Filion, Canada, 316.70.
9. Tonia Couch, Britain, 311.00.
10. Victoria Lamp, United States, 301.20.
11. Alejandra Orozco, Mexico, 298.15.
12. Laura Marino, France, 289.70.
Synchronized Swimming: Duet Free1. Russia (Svetlana Kolesnichenko and Svetlana Romashina), 97.680 points.
2. China (Jiang Tingting and Jiang Wenwen), 95.350.
3. Spain (Ona Carbonell and Margalida Crespi), 94.990.
4. Ukraine (Lolita Ananasova and Anna Voloshyna), 92.620.
5. Japan (Yumi Adachi and Yukiko Inui), 91.620.
6. Italy (Linda Cerruti and Costanza Ferro), 89.170.
7. Greece (Evangelia Platanioti and Despoina Solomou), 87.980.
8. Canada (Emilia Kopcik and Stephanie Leclair), 87.370.
9. Britain (Olivia Federici and Jenna Randall), 87.180.
10. France (Laura Auge and Margaux Chretien), 86.620.
Water Polo:Women's Preliminary Round:
Group A
Russia 12, Netherlands 12
Spain vs. Uzbekistan (late)
Group BAustralia 16, South Africa 1
China vs. New Zealand (late)
Group CUnited States 16, Britain 4
Canada 8, Greece 8
Group DHungary 18, Kazakhstan 7
Italy 13, Brazil 5