The £2 million account was previously held by Maxus and Seiko did not call a competitive pitch involving other agencies.
UM had already handled Seiko’s media business in France and Belgium.
As well as the Seiko brand of watches, the company produces Pulsar and Lorus watches. In the UK, Seiko distributes Ted Baker and Kenneth Cole watches.
The Seiko brand has heavily featured Novak Djokovic, the world's number one tennis player, in its recent advertising since hiring the Serbian athlete as its brand ambassador in 2014.
Bretton Essex-Evans, the group marketing manager at Seiko UK, said: "We selected UM London for our media planning and buying duties because they showed a great understanding of our brand and collections, and how to best connect them with our target audiences. We’re excited to see how UM will develop our approach to media over the coming months and beyond."