The Cairo-based institution hailed the formation of the 34-nation alliance announced by Saudi Arabia as "historic", and said it hoped it would defeat the "evils of terrorism".
"This was an urgent demand of the people of Islamic countries who have suffered more than others from this black terrorism," Al-Ahzar said in a statement.
"Al-Azhar calls on all Islamic countries to join this coalition to counter terrorism... That has committed horrible crimes indiscriminately."
The 34 countries participating in the coalition belong to the Jeddah-based Organisation of Islamic Cooperation.