"India has lost a cherished friend and will always value His Majesty's personal contribution to the strengthening of the close relationship between the Republic of India and the Kingdom of Thailand," he wrote in the letter.
Mukherjee said he was deeply saddened to learn of the demise, last evening, of the King.
"In his passing, Thailand and the world have lost a sagacious and dedicated national leader. His Majesty played a central role in shaping modern Thailand and in consolidating its relations with friendly nations around the world," he wrote in the condolence message.
"He remained devoted to the balanced advancement of his country and rightly stressed environmental and social responsibilities as equally important factors of economic progress. The people of India, who have long admired his charismatic personality, share the grief of the people of Thailand," Mukherjee wrote.