The shooting happened at the Church of Santa Maria during Sunday mass at around 11:40 local time (08:40 GMT). Turkey’s interior minister later said on X, formerly Twitter: “The two mur-der suspects have been captured,” without giving further details on the motive of the attack.
The Islamic State group has claimed responsibility for the attack.
In a post on Telegram the group said two of its fighters had carried out the attack and then fled to safety.
CCTV released by Turkish media shows the moment of the attack. Worshippers were on their feet, facing the altar, when two gunmen came into the back of the church.
They appeared to be following a man who had just come in. After he was shot and fatally wounded, the gunmen calmly left.
The 19th Century church sits behind high walls, close to a small fish market, by the Bosphorus on the European side of Istanbul. The streets around it have been closed off by police.
Inside the church, investiga-tors took statements from those who were there when the attackers struck.
The Turkish Interior Minister, Ali Yerlikaya, who visited the scene said that an individual identified only as CT had died in the attack.
Speaking to reporters outside the church, Istanbul governor Davut Gul said the victim was a Turkish national and that no one else was hurt. He said the atta-ckers only fired at one person.
It was not immediately clear what the motive for the attack was or why the victim was targeted.