The Egyptian men were beheaded by the militants group last week.
Now the men will be commemorated in the Coptic Church's calendar.
Pope Tawadros II announced that the names of the martyrs will be inserted into the Coptic Synaxarium and remembered every February 15.
Pope Tawadros II
That falls on the feast day of the Presentation of Jesus at the Temple.
Their martyrdom is similar to canonisation in the Latin Church.
Last week IS posted a video online which showed the men being beheaded by its militants.
The brother of two of the men expressed his pride in their actions in death, describing them as "a badge of honour to Christianity".
Speaking on a phone in programme on the Arabic TV station Sat-7, a caller named Beshir said the families are congratulating each other and are not in despair. He said: "We are proud to have this number of people from our village become martyrs.
"Since the Roman era, we as Christians are martyred, thus we have learned to handle everything that comes our way.
"Even the bible teaches us to love our enemies and bless those who curse us."
He also thanked Islamic State for not editing the video in which they were murdered. In it the 21 Copts declare their Christian faith.