The swearing-in ceremony took place at Amphorn Sathan Residential Hall in Bangkok in the presence of Their Majesties the King and Queen.
The PM was accompanied by his team of 34 ministers and deputies.
After the ceremony, Srettha told reporters that he would heed His Majesty's teachings as a standard while working.
He reiterated that his administration would serve as the people's government that would work tirelessly in the interests of the public.
He vowed to promote equality among all citizens in order to increase Thailand's prosperity.
When asked if he would personally head the government’s economic team, Srettha said that he would need to discuss the matter with other ministers first. He said “all ministers understand and are aware of the problems that the people are facing”.
“We are sincere and cognizant of the economic and conflict issues affecting citizens. The government would tackle each of these issues equally and work to move forward and find solutions as soon as possible,” he added.
The prime minister declared that his government would not have a "honeymoon period" and it would start working right away.
Srettha is set to hold a special meeting with his Cabinet on Wednesday to discuss the declaration of policies to Parliament, set for September 11-12.
After the government announces its policies to the legislature, the first formal Cabinet meeting will be convened. The agenda would include holding a referendum to amend the present Constitution and lowering energy costs, as promised by the Pheu Thai Party.
Srettha said that such plans would be explained on September 11–12.