The protests will likely undermine investor confidence and an already fragile growth outlook for next year, a credit negative factor for Thailand, according to Moody's Investors Service.
Thailand is currently rated Baa1 with a stable outlook by the rating agency.
Although the protests have mostly been peaceful, four people were killed and dozens wounded during clashes on Saturday night.
"Prolonged or escalating protests will adversely affect foreign investment and tourism, and exacerbate delayed public infrastructure investment, which will weigh on Thailand's future growth in 2014 and beyond," Steffen Dyck, an assistant vice-president, said.
While Thailand's credit fundamentals are still good, weaker growth will negatively affect the fiscal balance and push debt ratios.
Since US tapering fears emerged in May this year, emerging-market countries with relatively weak policy frameworks or external current account and domestic budget deficits have had more volatile capital flows.
Thailand has become more vulnerable to diminished investor confidence because it has these twin deficits, Moody's said, adding its foreign exchange reserves have declined since May.
Revised balance of payments data released in late September showed a current account deficit equal to 0.4% of GDP in 2012 as opposed to a small surplus previously.
During January to September, Thailand's current account deficit was $6.1 billion, up from a $3.4 billion deficit in the same period a year earlier.
Recent customs trade data point to continued weakness in export growth, with a trade deficit of $1.77 billion in October.
"We expect the current account to remain in deficit of about 1% to 1.5% of GDP throughout 2014 and the fiscal deficit at around 3% of GDP.
Thailand's growth momentum is also slowing. Third-quarter real GDP growth declined to 2.7% year on year from 5.4% in the first quarter.
This contrasts with South Korea, Malaysia and Singapore, which all reported stronger third-quarter annual growth rates, the rating agency said.