The appointment followed a tender process launched in October which attracted bids from six agencies.
MEC held the account for eight years.
Zenith has been awarded the contract from April 1 for an initial two year period with the option to extend annually for a further two, subject to performance.
STB assistant chief executive Lynette Pang said: “We look forward to working closely with Zenith as an agile and bold agency partner to help us create new possibilities and thrive in today’s dynamic media environment.
“We are grateful to all agencies who had put in the time and effort to participate in this process, and appreciate all the thoughtful submissions we received.
“We also thank our incumbent agency MEC for their valuable contributions and dedicated partnership for the past eight years.”
Zenith Singapore managing director Helen Lee said: “The travel industry has been the most disrupted through technology. As the ROI agency, we reimagined ROI for Singapore Tourism Board through personalisation as travellers connect to an experience journey.”
Carat Media Services Singapore, IPG Mediabrands (Singapore), Mediacom Singapore and OMD Singapore were the other agencies which had bid for the account.
The loss of STB will be seen as a major blow for MEC, with reports emerging yesterday that its Singapore managing director Sharon Soh had departed the agency.
Soh had been MD since mid 2012 after initially joining MEC in January 2003.
She rose through the ranks to become general manager in November 2005 and strategy director in 2008 before being handed the top job four years later.