The Office of the National Economic and Social Development Board stated that the downward revision was mainly from the drop in export values following the falling global oil and farm prices.
It cuts GDP growth forecast for this year to 3-4% from an earlier prediction of 3.5-4.5%.
It also expects Thailand to post only 0.2% growth in exports in 2015 from earlier projected 3.5%, while household consumption will expand by 2.3%.
The current account surplus for this year is expected to hit 3.9% of gross domestic product (GDP).
But NESDB secretary general Arkhom Termpittayapaisith said the weaker baht should have a positive impact on exports from the second quarter, As the Bangkok Post reported.
He said exports in the second quarter should be better than the first as there are more positive factors, citing accelerating government spending, increased private investment, tourism, and the weakening baht as reasons.
Exports fell for a third month in March, while consumer confidence dropped to a 10-month low in April.
The Bank of Thailand unexpectedly cut its key interest rate for a second straight meeting last month. The economy is recovering slower than expected, and exports are subject to greater downside risk, the monetary authority said in the minutes of its April 29 policy meeting.