The firm, which focuses on acid mine drainage in South Africa, is aiming to become the world leader in the sector.
The Bishopsgate team will be led by director Nick Rome. Account director Robyn Samuelson and account executive Giang Nguyen will also work on the account.
Rome said: ‘The Company has already made substantial progress towards implementing a sustainable solution to the problem of acid mine drainage in South Africa and we look forward to working closely with Watermark as it develops further.'
Peter Marks, chairman of Watermark, said: ‘It was important for us to find a PR agency which understands and has experience in natural resources, and which is as committed as we are to raising our profile and developing this exciting project.'