Patriarch Kirill I said Russian soldiers who die in the war will be absolved of "sins."
The Sunday sermin came days after Russia announced the mobilization of 300,000 troops.
Kirill is known to support Russian President Vladimir Putin and the invasion of Ukraine.
The leader of the Russian Orthodox Church compared dying in the war against Ukraine to an act of "sacrifice" and that doing so absolved soldiers of their "sins."
Patriarch Kirill I made his remarks on Sunday, days after Russia announced a "partial mobilization" of troops, and men continue to be seen fleeing the country to avoid the draft.
"Many are dying on the fields of internecine warfare," Kirill said, according to a translation by Reuters. "The Church prays that this battle will end as soon as possible, so that as few brothers as possible will kill each other in this fratricidal war."
"But at the same time, the Church realizes that if somebody, driven by a sense of duty and the need to fulfill their oath ... goes to do what their duty calls of them, and if a person dies in the performance of this duty, then they have undoubtedly committed an act equivalent to sacrifice. They will have sacrificed themselves for others. And therefore, we believe that this sacrifice washes away all the sins that a person has committed," he said, according to Reuters.
Kirill is known to be a supporter of President Vladimir Putin and of the invasion of Ukraine.
He previously justified the war as a fight against "excess consumption" and "gay parades" infiltrating Ukraine, according to The Orthodox Times. Kirill has also described Putin's leadership as a "miracle of God."
Putin announced on September 21 a "partial mobilization" of 300,000 military troops - an act seen as an escalation of Russia's war against Ukraine.
Almost immediately, one-way flight tickets out of Russia sold out and internet searches for "how to leave Russia" spiked in the country.
Lines of cars have packed Russia's borders with Georgia, Kazakhstan, Mongolia, and other countries, according to The Associated Press.