 Most had no warning of the 2004 tsunami until giant waves appeared |
The UN organisation, which has overseen the project, says the whole region can now receive and distribute warnings of possible tsunamis.
The system is in place 18 months after the devastating tsunami of December 2004 that killed more than 200,000.
The Pacific region has had a system for 40 years and others are planned for the Atlantic, Mediterranean and Caribbean.
Work unfinished
Koichiro Matsuura, director-general of the UN's scientific and cultural organisation, said the nations involved should be "justly proud of having done all this and much more".
There are 26 national tsunami information centres receiving information from 25 new seismographic stations.
There are also three deep-ocean sensors to detect and report tsunamis.
But Mr Matsuura warned the work was not yet finished.
He said the system would suffer if there was no coordination between the different nations.
"The open and free exchange of data and the full interoperability of national systems is absolutely crucial for success," he said.
Mr Matsuura also said that even a 100% successful warning system would be ineffective "if people do not know how to respond to the emergency".
The system is being overseen by Unesco's Intergovernmental Oceanographic Commission.
A massive earthquake beneath the ocean on 26 December 2004 sent giant waves crashing ashore in places as far apart as Indonesia, Sri Lanka, Thailand and Somalia.
The only warning most people had was the sight of the waves heading towards them. About 1.5 million people were left homeless in the region after the wall of water stripped away trees, houses and whole communities.
Reconstruction could take between five years and a decade.
