The number comes from studying relationships between the branches and leaves of the "family tree of life".
The team warns in the journal PLoS Biology that many species will become extinct before they can be studied.
Although the number of species on the planet might seem an obvious figure to know, a way to calculate it with confidence has been elusive.
In a commentary also carried in PLoS Biology, former Royal Society president Lord (Robert) May observes: "It is a remarkable testament to humanity's narcissism that we know the number of books in the US Library of Congress on 1 February 2011 was 22,194,656, but cannot tell you - to within an order of magnitude - how many distinct species of plants and animals we share our world with."
Now, it appears, we can.
"We've been thinking about this for several years now - we've had a look at a number of different approaches, and didn't have any success," one of the research team, Derek Tittensor, told BBC News.
"So this was basically our last chance, the last thing we tried, and it seems to work."
Dr Tittensor, who is based at the UN Environment Programme's World Conservation Monitoring Centre (Unep-WCMC) and Microsoft Research in Cambridge, UK, worked on the project alongside peers from Dalhousie University in Canada and the University of Hawaii.
The vast majority of the 8.7 million are animals, with progressively smaller numbers of fungi, plants, protozoa (a group of single-celled organisms) and chromists (algae and other micro-organisms).
The figure excludes bacteria and some other types of micro-organism. Read the full report here.
Source: By Richard Black, Environment correspondent, BBC News, U.K.
