Finnish President Sauli Niinistö signed the country's national Nato legislation on Thursday at the Government Palace in Helsinki.
Finland is now set to submit Nato membership-related documents to the US administration once the final two ratifications are received from Turkey and Hungary.
On 1 March, Finnish parliament approved the proposal for Finland's accession to Nato, legislation which the president signed on Thursday.
President Niinistö has said he wants Finland to be prepared to join the alliance as soon as Turkey and Hungary's ratifications are carried out
Last Friday, Turkish President Recep Tayyip Erdoğan announced that Ankara would begin the process of ratifying Finland's bid to join Nato.
The Hungarian parliament is expected to vote on Finland's membership next Monday.