The Schmitt family told Orlando's WKMG television they found the golden pyx, a box used to transport Eucharist wafers, in a 1715 wreck off today's Fort Pierce.
The vessel was carrying treasure from Spain's colonies in Latin America back to Spain, when it sunk during a hurricane.
It is not the first time the family has made a rich find: just last year the residents of Sanford, Florida, found gold chains and gold coins worth some $500,000 (€373,000), the US network said.
The Schmitt family owns a company in Florida — Booty Salvage Company — which specializes in treasure-hunting off the Florida region known as the Treasure Coast.
"We never expected to get rich off this. We do it for the love that we have for treasure hunting, for finding part of history,” Hillary Schmit told JCKonline after the family found its first treasure.
Pyxes, like the one the Schmitt family say they have found, were used by people who were ill or unable to go to church, to carry around sacramental bread.