The tiny blue and orange bird was discovered in a net by volunteer bird ringers at Bog Meadows.
Conservation charity Ulster Wildlife said it was the first recorded sighting of a young kingfisher at the reserve.
An adult kingfisher caught earlier in the summer was the first one found by researchers at the site in 20 years.
Bird ringers have been recording breeding birds at Bog Meadows since 1995.
The volunteers found the young bird last week during one of their morning visits to record, ring, and release breeding birds.
Aidan Crean, of preservation group Friends of Bog Meadows, said he was "absolutely thrilled" to find kingfishers at the west Belfast reserve, which sits beside one of the busiest roads leading into the city.
"Bog Meadows is one of the last remaining wildlife sites in the city, but due to its relative isolation and bustling location beside the M1, it's an unlikely spot to see these very vulnerable and shy birds," he said.
"[But] the fortunes of this wetland habitat have been transformed, thanks to conservation work carried out by Ulster Wildlife, creating an ideal location for kingfishers and other wetland-loving creatures to thrive."
Mr Crean said that while it was unlikely the birds were nesting at Bog Meadows, he hoped they would soon begin to raise their young there.
"There have been several sightings of adult kingfishers along the ponds and streams since the start of spring," he added.
"This is a good sign that there's plenty of fish to catch and, most importantly, clean water and good habitat."
Bird enthusiasts in Belfast have got themselves into a flutter after catching a rare glimpse of a fledgling kingfisher at a city nature reserve.