The leopard, which belonged to a rare sub-species known locally as "Gudhaphutuki", was sighted among bamboo trees in the Borduar Tea Estate by local people early this morning.
The locals informed forest department and police.
After an effort of nearly six hours, forest guards were able to tranquilise and cage the animal.
However, the leopard died while it was being shifted to the Assam State Zoo in Guwahati.
The exact cause of the animal's death is yet to be ascertained, but it was suspected that the dose given for tranquilising the leopard was higher than the stipulated safe level.