Wildlife photographer Steve Trewhella shot footage of the rays earlier in August, confirming the site is a nursery for the endangered species.
Marine awareness officer Julie Hatcher said rays reproduced very slowly.
Nurseries are found in only in a couple of places along the south coast.
Ms Hatcher said the trust had long been aware Studland Bay was a nursery area for rays, as in autumn and winter hundreds of egg cases, known as "mermaids' purses" are found along the shore.
Volunteers have been collecting the egg cases, and the majority are those of undulate rays.
However, the baby rays have not been photographed for a number of years: they are so well camouflaged, it is very difficult to spot them.
Ms Hatcher said: "They are a very beautiful animal.
"They have fantastic markings which looks a bit like Aboriginal art."
The nursery is particularly important because rays only lay a few eggs which take a long time to hatch, and take a couple of years to reach maturity, making the species very vulnerable to over-fishing.
Commercial fishing of undulate rays is prohibited, but they can be caught legally by recreational anglers.
Dorset Wildlife Trust says new footage of endangered undulate rays in Studland Bay is "exciting", and could add weight to its campaign to have it made a marine conservation area.