Sapo head Anders Thornberg said the agency has "never had a bigger and more complex task" and added Norway "has an increased military strategic importance".
While he did not explicitly talk about Russia, non-Nato member Sweden stationed permanent troops on the Baltic Sea island of Gotland in September.
The move was described as sending a signal after Russia's 2014 annexation of Crimea and its "increasing pressure" in the region.
Sweden, which is not a Nato member, is currently in the process of upgrading its military with a sharp hike in spending, and has urged local governments to prepare their civil defence infrastructure and procedures for a future war.
When presenting the agency's annual report, Mr Thornberg said it had investigated an undisclosed number of cases where people were suspected of giving out classified information to foreign governments.
The report came after Sweden reintroduced a military draft for both men and women over concerns about increased military action in the Baltic region.
Defence Minister Peter Hultqvist said the left-leaning government is reintroducing the draft because of a deteriorating security environment in Europe and around Sweden.
Under the newly approved plan, at least 4,000 18-year-olds could be called up each year, starting in January.
As in the current system, Swedes will still be able to volunteer for military service.
The country abolished the draft in 2010, when only men were eligible, because there were enough volunteers to meet the region’s military needs.