Though small compared with the value of debt Lisbon has issued through shorter-termed treasury bill auctions, the sale marks a step towards a full return to debt markets once the 78-billion-euro European Union/International Monetary Fund financing package ends next year.
The bonds, which mature in January 2018, were placed with a local institutional investor, a finance ministry spokeswoman said.
They will have a coupon of 3.75 percent and, from 2015 onwards, will pay the equivalent of six-month Euribor plus 250 basis points.
Five-year bonds now yield around 6 percent in the secondary market.
BNP Paribas interest rate strategist Ioannis Sokos said the minor private placement was unlikely to change the strategy initiated by Ireland and now followed by Portugal of swapping shorter-dated bonds for longer ones.
"We expect them to continue following the Irish example in swapping bonds, which went well and is a good thing to do, as well as lengthening T-bill maturities. Then, sooner or later, Portugal will be eligible for ECB's bond-buying programme.
I think these are the three main goals, not private placements."
On Oct. 3, Portugal swapped almost 3.8 billion euros in bonds maturing next year for debt due in 2015.
Some other analysts have said they expect Portugal to try a small syndicated bond issue in three three- or five-year segment in the next few months.
On Thursday the IMF warned of rising economic risks for Portugal but said it believes the country can finance itself in bond markets again from next year as planned.
Economists worry Portugal may fall into a recessionary spiral like Greece, saying sweeping tax hikes in next year's draft budget will drastically undermine consumer confidence. The draft may still face opposition in the courts.
Portuguese benchmark 10-year bond yields rose to around 8.08 percent on Friday from Thursday's settlement at 7.82 percent, which Sokos said was to a large extent due to IMF's warnings about rising economic and political risks.
The yield, however, is lower than the roughly 9 percent at which it started the month and well below January's record highs of over 17 percent.