Prime Minister Lee Hsien Loong met Indonesian President Joko Widodo in Sochi ahead of the Asean-Russia Commemorative Summit on Friday.
They happened to stay at the same hotel and on the same floor, Mr Lee said in a Facebook post.
He said he asked to "drop by for a quick catch-up" after he found out about the coincidence.
"We had a good four-eye meeting for 30 minutes before the summit started," said Mr Lee, using a term for a private meeting without staff members or reporters present.
The two leaders talked about the terrorism threat, and how their countries' agencies are cooperating to fight it together, said Mr Lee.
During the meeting, Mr Joko described his efforts to improve the business environment in Indonesia, including in Batam, Bintan and Karimun region.
Mr Lee said Singapore valued its economic ties with Indonesia, and Singapore investors appreciated the Indonesian government's support for their projects.
Both men will meet again later this year in Indonesia for a leaders' retreat, a meeting Mr Lee said they looked forward to.
He added that he hoped to visit Kendal, near Semarang in Central Java, where Sembcorp has a joint venture to develop a big industrial park.