Some 240,000 Indonesians visited the city-state during the month, or nearly 20% of all foreign arrivals, according to statistics from the Singapore Tourism Board.
China was in second place with 220,000, followed by India, Malaysia and Australia.
The Philippines, the fourth largest source of visitors in March, fell to sixth place.
So far this year Singapore has received 5.71 million international visitors, equivalent to 90% of the pre-Covid figure.
Last year there had been 13.6 million visitors.
It eyes 15-16 million this year.