President Halimah Yacob and Prime Minister Lee Hsien Loong have sent congratulatory letters to Sultan Hassanal Bolkiah of Brunei, who marks the 50th anniversary of his accession to the throne today.
PM Lee will attend this week's celebrations in Brunei.
In her letter, Madam Halimah said Brunei has achieved remarkable growth and success under Sultan Bolkiah's leadership.
He led his country to independence in 1984, and - amid challenges and uncertainties worldwide and in the region - built the peaceful and prosperous Brunei of today.
"Your Majesty's wise rule has enabled generations of Bruneians to live in peace and harmony, and to pursue their aspirations and reach their full potential," Madam Halimah wrote. "This is an enduring legacy."
Singapore and Brunei share a unique and special friendship, she added, built on close personal ties, mutual trust and a common understanding of the challenges and insecurities faced by small countries.
They collaborate in wide-ranging areas, including finance and trade, and defence and security, with both countries celebrating 40 years of defence cooperation last year.
Singapore is honoured to have hosted Sultan Bolkiah on four state visits here since his accession in 1967, said Madam Halimah.
During the Sultan's latest visit in July, Singapore and Brunei marked the Golden Jubilee of an agreement that allows both countries to accept their respective currencies.
"All these stand testament to the longstanding cooperation and level of trust between our two countries," Madam Halimah said.
"With Your Majesty's continued support, I am confident that Singapore and Brunei will continue to strengthen the firm foundation and special friendship between our two countries for many more decades to come."
Mr Lee, who will be in Brunei from today to Saturday to attend Sultan Bolkiah's Golden Jubilee celebration, also paid tribute to the longstanding ties between Singapore and Brunei in his letter.
These ties were founded on the close friendships between their leaders, and sustained by their shared strategic outlook on many regional and international issues.
The two countries have a multifaceted relationship, he said.
Singaporean companies are supporting Brunei's economic diversification, and both countries have regular public-sector exchanges as well as defence collaborations.
Mr Lee noted that Sultan Bolkiah has provided steadfast support of the Singapore Armed Forces' (SAF) training in Temburong.
"As small states with like-minded views, we also work closely in Asean to advance Asean unity and centrality," he added.
Mr Lee said he and his wife, Mrs Lee, have fond memories of visiting Brunei in 1992 for Sultan Bolkiah's Silver Jubilee celebration. Mr Lee was then deputy prime minister and minister for trade and industry.
"It was a majestic and memorable event. Twenty-five years on, we are deeply honoured to be invited to witness yet another historic milestone in Your Majesty's reign and in Brunei's history," he said, adding that they looked forward to personally congratulating Sultan Bolkiah.
Mr Lee's attendance at the celebration underscores the excellent and longstanding special relationship between Singapore and Brunei, the Prime Minister's Office (PMO) said in a statement.
Mr Lee will have an audience with the Sultan tomorrow, and attend the royal banquet at the Istana Nurul Iman.
During his trip, Mr Lee will also meet the Singapore community in Bandar Seri Begawan, and visit SAF troops training in Brunei.
He will be accompanied by Mrs Lee, and officials from the PMO and Ministry of Foreign Affairs.
In his absence, Deputy Prime Minister and Coordinating Minister for Economic and Social Policies Tharman Shanmugaratnam will be Acting Prime Minister.