The secretary-general of the bishop's conference Stefano Russo said the ruling "creates the preconditions for a culture of death in which society loses the light of reason".
The ruling is the latest development in the heavily Roman Catholic nation's long-running debate over end-of-life issues.
The constitutional court in Rome ruled in the case of a defendant who brought a friend, a well-known Italian DJ who was left quadriplegic by an accident, to Switzerland to die in an assisted suicide clinic.
Mr Russo said "such a strong ruling" should not precede action by the parliament.
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