The special prayers had began three months ago for speedy recovery and well-being of King Bhumibol amid increasing concerns about his health.
He died at a hospital in Bangkok on October 13 at the age of 88.
General Secretary of Mahabodhi Society of India K Siri Sumedh Thero said the special prayers continued even after his death for the peace of departed soul.
The prayer was held in front of his portrait that was kept along with Thai religious texts. Nearly 10,000 earthen lamps were also lightened at Sarnath in his memory.