"The Islamic State has seen success beyond the dreams of other terrorist groups that now appear conventional and even old-fashioned, such as al-Qaeda.
"It has energised tens of thousands of people to join it, and inspired many more to support it."
In all, between 27,000 and 31,000 foreign fighters from 86 countries have travelled to Iraq and Syria, The Soufan Group said, compared to a figure of around 12,000 foreign fighters in Syria when it last published a similar study in June 2014.
The largest number travelled to the two countries, across which IS controls a swathe of territory, from the Middle East and the Maghreb, with around 8,000 foreign fighters each.
Around 5,000 made their way from Europe, with a further 4,700 from former Soviet republics.
The Soufan Group added that between 20% and 30% of foreign fighters were returning to their home countries, creating major challenges for domestic security agencies as IS in particular looks to carry out an increasing number of attacks overseas.