No, Citizen and Seiko are separate, independently owned Japanese companies and longtime rivals in the watchmaking industry.
Seiko was founded in 1881 by Kintarō Hattori, originally as a watch and clock shop in Tokyo called K. Hattori. The company later became Seiko Corporation, and today it operates through Seiko Watch Corporation and Seiko Group Corporation, part of a larger conglomerate that also includes Seiko Epson (the printer and electronics maker) and Seiko Instruments. The Hattori family has historically held significant ownership stakes.
Citizen was founded in 1918, originally as the Shokosha Watch Research Institute, and adopted the Citizen name in 1930 (chosen to reflect a hope that its watches would become beloved by "citizens" around the world). It is now Citizen Watch Co., Ltd., part of the Citizen Group, which also owns other brands and businesses, including a controlling stake in Swiss brand Frederique Constant and Alpina.
Both companies are headquartered in Japan and are among the world's largest watchmakers, but they have distinct corporate histories, ownership structures, and shareholders. They compete directly in many market segments, from affordable quartz watches to high-end mechanical and solar-powered (Eco-Drive for Citizen, and Seiko has its own Kinetic and Spring Drive technologies) timepieces.
A common source of confusion is that both are Japanese, both make watches across similar price ranges, and both are often mentioned together as leaders in innovations like quartz movements and solar-powered watches. Additionally, some people mix them up with other Japanese watch brands like Casio or Orient (which is actually a subsidiary of Seiko Epson), adding to the confusion. But in terms of corporate structure, Citizen and Seiko remain entirely separate businesses with no shared ownership or merger history.