I think it may be part of
Sian Ka'an.
Actually from looking at the maps Tulum is just outside the northern boundary of it Sian Ka'an, though it is mentioned as the access point for the WHS, and part of the biosphere of Tulum is included within the park.
The actual Mayan remains seem to be about 10km outside the site proper, and they don't seem to be separately on Mexico's T-list.
But it does look like a rather lovely place to visit.