Television images showed police and an ambulance outside the ornate church.
Two men launched an armed attack on an Italian church in Istanbul during a religious ceremony on Sunday, leaving one person dead, Turkey's interior minister said.
The attack occurred at around 11:40 (0840 GMT) at the Santa Maria church in the Sariyer district of Istanbul and was carried out by two masked men, Ali Yerlikaya said on social media.
He added that an investigation had been launched.
Television images showed police and an ambulance outside the ornate church.
The minister said an individual identified only as CT -- who was among those attending Sunday's service -- had died after the armed attack.
"We strongly condemn this vile attack," Yerlikaya said.
The motive for the attack was not immediately clear.
In December, Turkish security forces detained 32 suspects over alleged links with Islamic State jihadists who were planning attacks on churches and synagogues, as well as the Iraqi embassy.
IS extremists have carried out a string of attacks on Turkish soil, including against a nightclub in Istanbul in 2017 that left 39 people dead.