The signing was part of the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) commission meeting being held in New Zealand.
Ministers from member countries will meet later on Sunday to discuss a range of topics, including how to move forward with new applications and a review of the agreement itself.
Britain's Business and Trade Secretary, Kemi Badenoch said at the signing that her country was delighted to become the first new member of the CPTPP.
"This is a modern and ambitious agreement and our membership in this exciting, brilliant and forward looking bloc is proof that the UK's doors are open for business," Badenoch said.
The British government still needs to ratify the agreement.
The CPTPP is a landmark trade pact agreed in 2018 between 11 countries including Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam.
Britain will become the 12th member of the pact that cuts trade barriers, as it looks to deepen ties in the Pacific after its exit from the European Union in 2020.
China, Taiwan, Ukraine, Costa Rica, Uruguay and Ecuador have also applied to join the CPTPP.
New Zealand Prime Minister Chris Hipkins said the road to bringing Britain into the agreement had been long and at times challenging, but having major economies inside the partnership would bring the Atlantic to the Indo-Pacific in a way that strengthened the rules-based trading system in the region.
