He was speaking after meeting the woman's widowed husband on the Greek island of Lesbos over the weekend, as he was visiting refugees on the island.
The unnamed Muslim man told the pontiff his late wife refused to renounce her religion to terrorists, who then slit her throat.
The Pope said: "He is Muslim, and he told me that he married a Christian girl.
"They loved each other and respected each other.
"But unfortunately the young woman's throat was slashed by terrorists because she didn't want to deny Christ and abandon her faith."
Refugees are not numbers, they are people who have faces, names, stories, and need to be treated as such.
— Pope Francis (@Pontifex) April 16, 2016
Pope Francis also decided to bring back three Muslim refugee families with him back to the Vatican.
Arriving on Saturday night to stay with the Catholic charity Sant'Egidio, they were given rose bouquets and traditional Syrian food served by refugees who'd already arrived in the city. They also helped with translation where possible.
One refugee taken in by Pope Francis, married mother-of-one Nour Essa, said: "We didn't believe it. He is a very kind man, he is very nice.
"We said to him thank you.
"There is no Muslim leader or Arabic leader who has done the same thing with us."
