[[Berkas:Eisai_Head_Office.jpg|jmpl|Kantor pusat Eisai di Tokyo]]
Eisai Co., Ltd. berkantor pusat di [[Tokyo]], Jepang, sementara Eisai Inc., anak perusahaanperusahaannya di Amerika Eisai Inc.Serikat berkantor pusat di Woodcliff Lake, [[New Jersey]]. Eisai memiliki kantor pusat [[Riset medis|penelitian medis]] di Woodcliff Lake, New Jersey serta lokasi di [[Jepang]], [[Britania Raya|Inggris Raya]], Research Triangle di [[Carolina Utara|North Carolina]], dan [[Massachusetts]] di mana Eisai Research Institute dan Andover Innovative Medicines Institute berbasis.<ref name="carroll2016">{{Cite news|url=https://endpts.com/eisai-launches-research-division-in-massachusetts-aimed-at-alzheimers-io-and-autoimmune-diseases/|title=Eisai launches biotech division in Massachusetts aimed at Alzheimer’s, I/O and autoimmune diseases|last=Carroll|first=John|date=15 September 2016|work=Endpoint News|access-date=4 November 2016}}</ref><ref name="genengnews2016">{{Cite news|url=|title=Eisai Launches Facility to Advance Precision Medicine|last=<!--Staff, no byline-->|first=|date=15 October 2016|work=[[Gen. Eng. Biotechnol. News|Genetic Engineering & Biotechnology News]]|department=News: Industry Watch|page=8|type=Paper}}</ref>
Perusahaan ini memiliki pabrik di Jepang, [[Carolina Utara|North Carolina (AS)]], [[Maryland|Maryland (AS)]], [[Kota Bogor|Bogor (Indonesia)]], [[Suzhou|Suzhou (Tiongkok)]], [[Kota Tainan|Tainan (Taiwan)]], [[Visakhapatnam|Visakhapatnam (India)]] dan Hatfield, Hertfordshire (Inggris). Eisai memiliki kantor pemasaran di 19 negara Eropa serta Asia-Pasifik.
