Esmond Bradley Martin, 75, was found with a stab wound to his neck at home in the capital Nairobi on Sunday.
The former UN special envoy for rhino conservation was known for his undercover work establishing black-market prices.
The US citizen had recently returned from a research trip to Myanmar.
Bradley Martin was in the process of writing up his findings when he died, reports the BBC’s Alastair Leithead from Nairobi.
His wife found him in their house in Langata. Police are investigating the circumstances but suspect it was a botched robbery.
Our correspondent says Bradley Martin had spent decades risking his life to secretly photograph and document the illegal sales of ivory and rhino horn, travelling to China, Vietnam, and Laos to pose as a buyer – helping to find out the level of black market prices.
He first went to Kenya from the US in the 1970s when there was a surge in the number of elephants being killed for their ivory.
Image copyright AFP
Image caption Conservationists believe that the ivory trade is largely responsible for the world’s declining elephant numbers
His work on illegal wildlife markets helped pressure China to ban the rhino horn trade in the 1990s, and domestic sales of ivory, which came into force this year.
Fellow conservationists have been paying tribute to him on social media.
Skip Twitter post by @paulakahumbu
2/3 Esmond was at the forefront of exposing the scale of ivory markets in USA, Congo, Nigeria, Angola, China, Hong Kong, Vietnam, Laos and recently Myanmar. He always collaborated with Save the Elephants and worked with many of us generously sharing his findings & views.
— Dr. Paula Kahumbu (@paulakahumbu) February 5, 2018
Always sharply dressed with a colourful handkerchief falling from his top pocket, Esmond Bradley Martin would immediately cut to the chase, honing in on the latest issue that was consuming him.
He was a well-known and highly respected character in the conservation community – passionate and unwavering in his efforts to crack down on illegal wildlife crime.
In a major report last year from Laos, he and his colleague Lucy Vigne established that the country had the world’s fastest growing ivory trade.
They risked their own safety staying at a Chinese casino inhabited by gangsters and traffickers in order to visit the illegal markets and find out the latest prices by posing as dealers.
His life’s work was combating the illegal trade of wildlife and he produced a huge body of highly respected research and investigative reports.
He will be a huge loss to the international conservation community.