The Board of Investment said on Tuesday it approved applications worth about 800 billion baht last year, missing its target of 1.4 trillion baht by nearly 43%, but up from 724.7 billion baht in 2014.
The investment agency did not meet its target in 2015 as some applications were submitted prematurely and others needed the input of several government agencies, Hirunya Suchinai, the BoI's secretary-general told reporters.
"We did not meet the target because sometimes these are large projects and require approval from committees," Mr Hirunya said, adding that the agency would try to clear its backlog in 2016.
Since taking power in a May 2014 coup, the country's military rulers have struggled to kick-start Southeast Asia's second-largest economy. The economy grew 1% in the third quarter, up from 0.6% in the same period last year.
The government has accelerated approvals for investment projects and offered various incentives to help support the struggling economy as exports and domestic demand remain sluggish.
The central bank expects zero export growth this year partly due to a slowdown in China's economy. Tourism, which accounts for about 10% of GDP, is expected to help prop up the economy.
The investment agency replaced previous policies in 2014 by gearing its incentives to more value-added sectors.
The BoI said 1,038 project applications were recorded in 2015 versus 3,469 in 2014.
Foreign direct investment from Japan in 2015 remained the highest in Thailand, with total investment approved from Japan valued at more than 144 billion baht for 426 projects.
Singapore came a distant second with 122 projects approved valued at 39.9 billion baht between January and November.