Which company created the Metroid series?

Nintendo created the Metroid series. The original Metroid was developed and published by Nintendo, with Nintendo R&D1 leading development and Intelligent Systems assisting with programming.

The first game launched in Japan for the Family Computer Disk System in 1986 and later reached the Nintendo Entertainment System. Its design combined platforming, shooting, and nonlinear exploration, drawing inspiration from games such as Super Mario Bros. and The Legend of Zelda. The result established a science-fiction adventure built around discovering upgrades and using them to reach previously inaccessible areas.

Several important Nintendo creators shaped the series. Gunpei Yokoi produced the original game, Satoru Okada directed it, Yoshio Sakamoto contributed to character design and later directed or supervised many entries, and Makoto Kano wrote the scenario. Nintendo continued publishing the franchise while different internal and external teams developed later games, including Retro Studios for the Metroid Prime series and Team Ninja for Metroid: Other M.

The answer is Nintendo rather than Nintendo R&D1 because the question asks which company created the series. R&D1 was Nintendo's development division, not a separate company. Other companies helped develop individual installments, but Nintendo owns and originated the Metroid franchise.