The Regional Comprehensive Economic Partnership (RCEP) is made up of 10 Southeast Asian countries, as well as South Korea, China, Japan, Australia and New Zealand.
The pact is seen as an extension of China's influence in the region.
The deal excludes the US, which withdrew from a rival Asia-Pacific trade pact in 2017.
Negotiations over the new RCEP deal began in 2012 and it was finally signed on Sunday on the sidelines of a meeting of the Association of Southeast Asian Nations (Asean).
"Under the current global circumstances, the fact the RCEP has been signed after eight years of negotiations brings a ray of light and hope amid the clouds," said Chinese Premier Li Keqiang, describing the agreement as "a victory of multilateralism and free trade".
India was also part of the negotiations, but it pulled out last year over concerns that lower tariffs could hurt local producers.
Signatories of the deal said the door remained open for India to join in the future.
Members of the RCEP make up nearly a third of the world's population and account for 29% of global gross domestic product.
The new free trade bloc will be bigger than both the US-Mexico-Canada Agreement and the European Union.
(Excerpts : BBC News / Pic : Xinhua)