The two Primary 5 pupils from Fernvale Primary School had set off for school as usual at about 7am on Monday.
But when they were not home at the usual time, their parents checked with the school and found that both girls had not attended school that day.
The boy was last seen at 7am at Toa Payoh Int, which is near their school.
On Tuesday, the police put up an appeal for information on Twitter, carrying photos of the boy. They were found at about 3.30pm yesterday.