Researchers say that for some children, the effects may be felt throughout their lifetimes unless they get the help they so urgently need.
A study found 25.9 per cent of children aged between three and five suffer from symptoms including vertigo, nausea and headaches, with some exhibiting worrying behaviour such as violence or withdrawal.
Youngsters were scarred by losing friends, seeing their homes destroyed, by separation from parents or by the sight of the huge wall of water that crashed ashore, the study team said.
These could include developmental disorders and learning disabilities, which would affect academic achievement and employment prospects, "as they may have trouble in communicating with other people due to the influence of experiences related to the disaster", Kure told AFP.
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More than 18,000 people died when a 9.0-magnitude undersea earthquake sent a towering tsunami into Japan's northeast coast in March 2011.
The country's worst post-World War II disaster was compounded by reactor meltdowns at the Fukushima nuclear power plant, which sent tens of thousands of people fleeing from radiation.
They used an internationally recognised child behaviour checklist and met children between September 2012 and June last year.
The level of children who need psychiatric care is up to three times that seen in parts of Japan unaffected by the disaster -- for example, 8.5 per cent of children in Mie prefecture in central Japan need help.
"I was surprised at the percentage of children who need medical care. I didn't expect it would be this high," Kure said.
"I imagine there are lots of children, who seemingly are leading normal lives but show behaviour that needs a doctor's attention, for example, waking up suddenly at midnight or biting their nails.