The west end star, who acquired a reputation as a "love rat" after dating actresses such as Anna Friel, Tracy Shaw and Suzanne Shaw, will be seen on the Channel 4 show from next month.
Day will play Danny Houston, the best friend of nasty Brendan Brady.
His character is also the silent partner in the Chez Chez nightclub run by Cheryl Brady.
Day said yesterday: "Danny is the type of character any actor would kill to play. I've been a fan of the show for some time and I'm delighted to join the cast."