More than 200,000 people were killed when an earthquake beneath the ocean sent giant waves crashing ashore.
Places as far apart as Sri Lanka, Thailand and Somalia were affected by the disaster.
Worst affected was the Indonesian province of Aceh, closest to the quake epicentre, where more than two-thirds of the deaths occurred.
A minute's silence was held in the provincial capital Banda Aceh to mark the exact moment the first waves came ashore, and a siren then sounded, part of Indonesia's new tsunami warning system.
Further south in the western Sumatran town of Padang, the system was tested for the first time, with around 2,000 residents evacuated as part of the exercise.
"We knew it was just a drill," survivor Candra Yohanes, 55, told AP news agency. "Still, when I heard the siren, my heart was pounding so hard."
Speaking in Banda Aceh, Indonesian President Susilo Bambang Yudhoyono paid tribute to those who had tried to rebuild their lives over the past year.
"You have reminded us that life is worth struggling for," he said.
Tens of thousands of survivors are still living in tents and it is estimated that at least 80,000 new houses need to be built.
"I would like to ask the president for a house because right now it's in a bad condition," Marriatti, 39, told Reuters news agency. "I had to build a house by myself."
Train disaster
Sri Lanka has been paying tribute to more than 30,000 people who were killed on the island.
"I think you need to come back"
Around the island, small private ceremonies were held to mark the moment the waves struck.
Thousands of people lit coconut oil lamps on beaches on the southern coast after dark.
The government held the official ceremony at Peraliya on the southern coast, where more than 1,000 people died when a train was swamped by the incoming water.
Temple bells signalled the beginning of a two-minute silence at a ceremony, attended by President Mahinda Rajapakse.
Buddhist and Muslim priests chanted blessings at the site.
The BBC's Dumetha Luthra, in Peraliya, says the site of the train derailment has come to symbolise Sri Lanka's national devastation.
But she adds that a year on, the line has been reconstructed, the train is once again running and that all along the coast, while still remembering the dead, people are continuing their lives.
Tourist toll
Thailand has been remembering more than 5,000 people who lost their lives there in the tsunami, two in five of them foreign tourists.
Worst hit was the stretch of coastline at Khao Lak in southern Thailand, where local Thais and the foreigners who were caught up in the disaster bowed their heads in silent contemplation before laying flowers in memory of those who died.
Prime Minister Thaksin Shinawatra attended the ceremony, laying a foundation stone for a memorial.
"I think you need to come back," Swedish survivor Pigge Werkelin, who lost his two young sons and his wife in the disaster, told Reuters news agency.
"You need to go to the beach, you have to see children on the beach, you have to see everything... I must do it and then afterward I can put it behind me."
Around 1.5 million people were left homeless in the region after the wall of water stripped away trees, houses and whole communities, and reconstruction could take between five years and a decade.
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