Day had been interim product comms manager at Lilly for just under a year.
His new brief is to promote Eisai's corporate reputation and products across the Continent.
The firm has operations in five European countries - the UK, Germany, France, Spain and Italy - and employs around 550 people across the region.
Its brands include Aricept (for Alzheimer's disease), Pariet (for acid-related disorders) and Zonegran (a migraine treatment).
The appointment of Day, who is based in Hammersmith, west London, concludes a recruitment process that kicked off earlier this year to fill the newly created comms position (PRWeek, 11 March). Eisai's previous corporate PR efforts in Europe have been handled by individuals seconded to London from Tokyo.