The country was ranked 137th in the second edition of the report published in 2013.
Switzerland topped the ranking while Iceland bagged the second place in the 2015 report.
The World Happiness Report reviews the state of happiness in the world today and shows how the new science of happiness explains personal and national variations in happiness.
GDP per capita, social support, healthy life expectancy, freedom to make life choices, generosity and perceptions of corruption are among factors that considers when preparing the report.
The first report was published in 2012, the second in 2013, and the third on April 23, 2015.
In the third World Happiness Report, now encompassing 158 nations, Denmark has slipped to third.
The UK is 21st; the US is 15th, with New Zealand 9th and Australia 10th.