The Nordic country closed the border in mid-December after nearly 1,000 migrants had arrived without a visa through its border crossings with Russia since August.
Finland has since extended the closure several times, with the last extension set to expire on April 14.
“The decision will remain in force until further notice, but no longer than is necessary,” the government said in a statement on Thursday.
In addition, “the border crossing points for maritime traffic at Haapasaari, the port of Nuijamaa and Santio will be closed to leisure boating”, starting April 15 to prevent the “possibility that instrumentalised migration could expand to maritime traffic”.
As of April 15, it will only be possible to apply for asylum at other maritime border crossing points or airports.
Moscow warned of “countermeasures” after Finland joined the Western defence alliance Nato in April last year, reversing its decades-long policy of military non-alignment.
Finland school shooting was likely planned, police say
Finland school shooting was likely planned, police say
In November, Finland initially closed four of its eight border crossings with Russia, and shut the rest by the end of the month.
Although Helsinki then partially eased the closure, on December 14 it once again decided to close the entire border.
According to Finnish authorities, reopening the border could pose a threat to national security and public order.
“Finnish authorities see this as a long-term situation. We have not seen anything this spring that would lead us to conclude that the situation has changed meaningfully”, Minister of the Interior, Mari Rantanen, said in a statement.
Rantanen added that hundreds and possibly thousands of people remain close to Finland’s border on the Russian side.
The Finnish government is also preparing a bill which would enable it “to restrict the reception of applications for international protection” in limited areas.