Defence Minister Jussi Niinistö inked the deal making Finland a member of the UK’s Joint Expeditionary Force on Friday.
Niinistö formalised the arrangement in Stockholm, where Sweden also signed up for the multinational force.
The aim of the JEF is to develop rapid reaction military performance. Other current members are Denmark, Estonia, Latvia, Lithuania, the Netherlands and Norway — all NATO members.
The UK invited Finland to join the group in February this year. At the time the Finnish Defence Forces said that the collaboration would not involve any new hardware purchases, since the JEF seeks to use and develop existing assets.
Niinistö: This is not a military alliance
The JEF is not staffed by permanent troops. Rather, members contribute resources during crisis situations.
Defence Minister Jussi Niinistö said that the force is not a mini-NATO.
"Finland will not get any kind of security guarantee, this is not a military alliance," Niinistö pointed out.
He noted that the joint force would assist Finland in a crisis.
"The crisis could be an epidemic, a natural disaster or even a military matter," Niinistö added.
JEF troops have assisted in areas such as Sierra Leone, during its Ebola crisis. At the time, British troops received backing from Norwegian and Dutch forces.
Edit: Updated at 5.07pm to include comments from Defence Minister Jussi Niinistö.