As always seems to be the case, different studies provide different results -

According to a new survey, Thailand ranked 6th most unequal country in the world in terms of wealth distribution.
Credit Suisse has released a global wealth report that indicated that up to 75 percent of the national wealth is held by those in its top 10 percent. The figure increases from that in 2013 at 73.8 percent.

The rate of unequal distribution of wealth accelerated during 2007-2014, with the share of wealth held by the top 10 percent standing at 69.3 percent in 2007.

http://thainews.prd.go.th/website_en/news/news_detail/WNECO5710200010004