The account, which is worth around £2 million in the UK, will cover brands including easyCruise, easyInternetcafe, easyHotel and easyCinema.
Initiative won the business in a pitch against the incumbent, PHD. The agency, which will handle the account from London, will focus on planning and buying in the UK, France, Germany and the US.
EasyGroup, which has launched a range of businesses under the umbrella "more value for less", called the pitch in August. It is run by Stelios Haji-Ioannou and comprises 15 businesses, nine of which launched in the past year.
Jerry Hill, Initiative's executive vice-president for Europe, the Middle East and Africa and group chief executive for the UK and Ireland, said: "Agencies are defined by their clients. To partner with such an innovative and entrepreneurial brand such as easyGroup is very exciting."