UK native Nichols will return in May to his home country from Montreal, where WADA is based.
He said: "It has been a hugely fulfilling four years leading the media relations at a time when WADA has been in the public eye like few other organisations worldwide.
"The integrity of sport has been under real threat these past couple of years, but day in, day out, WADA has remained committed to protecting the rights of clean athletes, promoting clean sport and delivering the level playing field that athletes and indeed the broader public so deserve."
Nichols said he would seek a "new challenge" in the UK.
Prior to WADA, he has worked for the Association of Tennis Professionals, the Women's Tennis Association's Dubai Tennis Championship, F1 team Lotus Renault GP, and the agency Influence Associates.
Created in 1999, WADA's funding is sourced equally from the Olympic Movement and governments of the world.
In 2015, the most recent year for which an annual report is available, it had 81 staff, with 68 of those based in Montreal. In 2015, it received an average of 100-150 media inquiries per month, the report states.
Nichols is part of a six-person team led by comms director Catherine MacLean.
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