BMB won the business in a pitch that kicked off in July. The final stage also involved VCCP and Anomaly Amsterdam. The process was run through AAR.
BMB will create an above-the-line campaign to run in markets where Merlin has Legoland parks – the UK, Denmark, Germany, California, Florida and Malaysia. The work will launch ahead of the peak Easter season in 2014.
The campaign will encourage visitors to stay longer at the Legoland theme parks, which are based around the Lego building brick toys.
DLKW Lowe has worked with Legoland on a project basis since 2005 but was not involved in this recent pitch process.
Dan Gregson, the managing director of BMB, said Legoland was a "unique and vibrant brand".
He said: "There really is no other theme park concept like it."
Merlin, which also owns Alton Towers, has recently added hotels, water parks and other additional attractions such as its Sea Life aquariums to the Legoland parks, in a move to encourage visitors to use them as short break destinations.
Merlin Entertainments floated on the stock market in November and raised at least $1.5 billion (£920 million).