A recent WIN/Gallup poll found that China is the most atheistic nation in the world. 47 percent of respondents said they did not belong to any religion.
This high presence of atheism is not unusual, as results from the poll indicated a clear decline in religiosity worldwide.
China has had a rather turbulent relationship with religion since the middle of the last century. In 1949 the Communist party, in line with Marxist ideology, declared China an officially atheist state. Things turned for the worse during the Cultural Revolution when any form or representation of religion faced fierce condemnation due to it being perceived as symbolic of Western colonization.
Today, Taoism, Buddhism and traditional folk religions are predominant, while Christianity and Islam, at 4 percent and 2 percent of the total population, have a minor presence. (From Shanghaiist)
In 2011, 67% of Japan claimed they were irreligious, and what's more atheism is not the same as being non-religious. So I don't think this thread title is accurate. China is probably high on the list, but not likely on top.
The percentage may be significantly greater (up to 80%) or smaller (down to 30%) in reality, because some Chinese define religion differently. Some Chinese define religion as practicing customs (which may be done for cultural or traditional reasons), while others define it as actually consciously believing their religion will lead to post-mortem salvation/reincarnation. According to the surveys of Phil Zuckerman on Adherents.com in 1993, 59% (over 700 million) of the Chinese population was irreligious and 8% – 14% was atheist (from over 100 to 180 million) as of 2005.