In 2008, a total of 408,600 people registered for jobs in the first 11 months of the year, up 2.9 percent from a year earlier, the institute said in a report.
From September to November, a monthly average of 63,700 people lost their jobs, a rise of 19.7 percent as compared with the previous corresponding period.
Analysts said the rising jobless rate was due to the worsening economic situation in the country.
On Nov. 19, the Bank of Portugal said the country's economy faced a "marked deceleration" in 2008, and the growth of the gross domestic product (GDP) would not exceed 0.5 percent.