President Tony Tan Keng Yam said Mr Sirisena's victory "is a testament of the faith that the people have in your leadership", the Straitstimes Asia reported.
Adding that Singapore has consistently supported national reconciliation, peace and progress in Sri Lanka, he said: "We hope that the positive momentum in Sri Lanka's reconstruction and development efforts will continue."
Prime Minister Lee Hsien Loong, in his congratulatory letter, said he was confident that under Mr Sirisena's leadership, "Sri Lanka will continue on its current path of peace and progress and grow to greater heights".