Tidalwave, which won the account from incumbent Kingpin following a three-way pitch, will be tasked with creating direct marketing campaigns that target small to medium-sized businesses and professional consumers.
The campaigns will focus on highlighting the benefits of Citrix Online's services, which allow customers to access software on demand, over the internet.
Simon Presswell, EMEA managing director of Citrix Online, said: "Recent mergers and acquisitions in this marketplace have created an opportunity for us, especially in the SMB space."
Paul Cash, managing director of Tidalwave, said: "In the US, Citrix Online has had unprecedented success and is now firmly established as the leader in the online access and collaboration market for SMBs.
"Working together, we will put in place a creative, high impact programme that will replicate this success throughout the European market."