"I resign in contention with the decision to send the marines back to India. The misgivings I expressed had no effect on the decision taken," he told parliament.
"I am resigning because for 40 years I have maintained, and still maintain, that the reputation of the country, the armed forces and Italian diplomacy, should be safeguarded," he said.
The Italian government sent Massimiliano Latorre and Salvatore Girone, accused of shooting dead two fishermen off the coast of the southwestern Indian state of Kerala, back to New Delhi on Friday, having previously opposed the trial in India.
What the stars mean:
★ Poor ★ ★ Promising ★★★ Good ★★★★ Very good ★★★★★ Exceptional