Prime Minister Lee Hsien Loong has written to Thai Prime Minister Prayut Chan-o-cha to express his condolences on the death of former Thai premier and Privy Council president General Prem Tinsulanonda.
Gen Prem, who was Thailand's prime minister for more than eight years in the 1980s, died of heart failure on May 26 at age 98.
Describing Gen Prem as a "capable and immensely respected leader", Mr Lee said that as premier, the general had led Thailand through a period of democratic development, holding three elections under his watch and progressively including more elected ministers in each of his five Cabinets.
As the president of the Privy Council since 1998, Gen Prem continued to be a loyal and trusted source of advice and guidance to the king, and provided wise counsel to successive military and civilian governments, said Mr Lee.
Mr Lee said the region also benefited from Gen Prem's leadership, adding that the general's time as premier coincided with five Asean countries coming together to resolutely oppose Vietnam's invasion of Cambodia.
"General Prem was resolute in not accepting this fait accompli. Supported by his able Foreign Minister, Air Chief Marshal Siddhi Savetsila, General Prem worked with Asean partners to support the resistance forces of the Coalition Government of Democratic Kampuchea from Thai territory, and to oppose the Vietnamese occupation in international forums.
"This effective collective resistance prevented a military invasion and regime change from being legitimised, and protected the security of other South-east Asian countries. Eventually the invasion forces withdrew, a peace settlement was signed, and internationally supervised elections were held to elect a new Cambodian government.
"This decisively shaped the subsequent course of South-east Asia. It paved the way for Vietnam, Cambodia and Laos to join Asean, as partners in promoting the region's peace and development," Mr Lee wrote.
Mr Lee said Gen Prem was also a good friend of Singapore, for whom the late founding prime minister Mr Lee Kuan Yew had deep respect as a statesman.