Säpo chief Anders Thornberg said that, of those 300, 40 have so far been killed in combat, and around 125 are still fighting in Syria and Iraq.
“That is quite a large number,” Hans Brun, a researcher on counterterrorism at Kings College in London told Sweden's TT news agency.
“Compare, for example, with the Second World War when there were only about 180 Swedish volunteers in the SS.”
“Compared to similar countries, a disproportionate number have traveled from Sweden,” agreed Thornberg.
Thornberg continued: “There is a violent Islamist environment in Sweden that influences people to go. The core of about 200 people who support the logistics, money and recruitment has been fairly successful here.”
“We've never seen anything like it. Before Syria and Isis we had maybe 40 travellers to Yemen, Afghanistan, Pakistan and other countries. Then we had a fair chance to keep track. Now we have 300 in three years,” said Thornberg.
Säpo has a list of 115 Swedes who have returned home.
“We do not think there are 115 potential terrorists, but those who come home are sometimes treated like rock stars and become role models for other young people. We are facing a historic challenge,” underlined Thornberg.
He noted that Säpo monitoring of the returning extremists is an important part of the efforts to prevent a terror attack in Sweden.
“Our task is to identify them, to sort them. There are one or two who are terrorists among them and we must deal with them,” said the Säpo chief.
There are growing concerns in Sweden about the number of young people from the Nordics who are coming into contact with extremist groups.
Last month a teenage boy who disappeared from southern Sweden in the spring and was thought to have joined Isis, returned to Sweden.
In August, The Local reported that a man from central Sweden who is believed to have been fighting alongside Isis had reportedly been killed.
The Swede, who is thought to be in his 30s and who had been living in Örebro, is understood to be one of a group of three people from the area known to have travelled to the Middle East to join the radical Islamist group, also known as Isis and IS.
Earlier in August, a pregnant 15-year-old Swedish girl and her 19-year-old partner were captured by extremist Islamist fighters in Syria.
Sweden's Security Service, Säpo, previously said that up to 150 Swedish residents were known to have spent time in Syria or Iraq fighting for Isis or other radical organisations, with intelligence suggesting that at least 40 have died in the process.
Sweden is drafting new legislation that would ban its nationals from fighting in armed conflicts for armed extremist groups such as the Islamic State (Isis), the government announced in June.
The proposed ban would prohibit combat for terrorist organizations listed as such by the United Nations or European Union.