Prime Minister Lee Hsien Loong paid his respects to Thailand's late King Bhumibol Adulyadej at Bangkok's Grand Palace yesterday.
Accompanied by Mrs Lee, he signed a condolence book and laid a wreath at the palace, where the King's body lies while funeral rites are being completed.
In a Facebook post yesterday, Mr Lee wrote: "His Majesty had a long and benevolent reign. He was a great leader who shaped Thailand's development and improved his people's lives.
"He will also be remembered for fostering the enduring friendship between Thailand and Singapore. May he rest in peace."
An earlier scheduled meeting with Thai Prime Minister Prayut Chan-o-cha was "cancelled due to last-minute scheduling difficulties", the Ministry of Foreign Affairs said in statement yesterday.
Mr Lee is the first Asean leader to have travelled to the Thai capital to pay his respects since the monarch died last Thursday. Malaysian Prime Minister Najib Razak, his Cambodian counterpart Hun Sen and Indonesian President Joko Widodo are expected to arrive in the coming days.
Grieving Thais have been pouring into the capital to bid their last farewell to the monarch, who reigned for 70 years and was regarded as a unifying figure in times of political turbulence.
His heir apparent, Crown Prince Maha Vajiralongkorn, has yet to ascend the throne as he has asked for time to mourn with the people. Standing in as regent is General Prem Tinsulanonda, who used to head the King's privy council.
At a seminar yesterday, Mr Prayut said: "His Majesty has not left us. He is still in the earth, the water, the air."
According to a report by Reuters news agency, he also asked Thais to remember their traditions.
"Our country is between two paths - one is failure, the second is that we can all help create history in Thailand," he was cited as saying.
"Let us not forget Thailand has had a monarchy for 700 to 800 years.
"We have managed to survive because of the monarchy. Though the world is changing, we must not forget our roots."