The Assumption Cathedral in Kozelsk is the main Orthodox cathedral of the Kozelsk diocese in the Kaluga metropolis. According to tradition, it was built on the site where a wooden church stood in 1238, where the last defenders of the fortress of Prince Vasily Kozelsky chose to burn alive rather than surrender to the Tatar-Mongols. Archive data from the Kaluga region indicates that the stone cathedral with a bell tower was built in 1824 by Archpriest Anchukovsky and the parishioners. However, in the "Description of Church Monuments of the Kaluga Province" from 1882, Count Roshefor mentioned a different builder, Znaukovsky.

The exact time of the stone church's construction is unknown. On the Kozelsk map from the Atlas of the Kaluga Governorate of 1782, the stone Assumption church is already marked on the territory of the former fortress. Next to it, a wooden church was depicted, possibly a chapel in honor of the Iveron Icon of the Mother of God.

Historian Leonid Kavelin mentioned in his "Historical Description of the Kozelsk Vvedenskaya Optina Monastery" that the cathedral survived the fire of 1777 and existed in the 18th century, being updated and rebuilt. In the 19th century, the cathedral housed a Gospel from 1663 with an inscription about its donation by the peasants of the village of Deshovka, the estate of Prince Yakov Cherkassky.

In the early 1920s, a choir of monks from closed churches of the Optina Monastery was formed in the cathedral. Among the participants were Father Geronty, Father Rodion, Father Vasily, Father Akakiy, and others. Archpriest Sakharov served in the church in 1854.

Bishop Mikhey (Alekseev), born in St. Petersburg, after serving in the naval department and becoming a widower, became a novice at the Optina Monastery. He served there until 1923, after which he moved to Kozelsk and served in the Assumption Cathedral.

The Venerable Nikon, known as a confessor, was born in Moscow and aspired to spiritual life from childhood. After the closure of the Optina Monastery in 1924, he settled in Kozelsk and served in the Assumption church. In 1927, he was arrested and spent three years in a camp, dying in 1931.

The cathedral was closed in 1928 and used as a drama theater, and during the war, as a German stable. In the 1950s, the building became a House of Culture, but by the end of the 20th century, it fell into disrepair.

In 2001, a resident of Kozelsk, V. I. Agulin, proposed to restore the cathedral. After the church was transferred to the diocese, restoration work began under the guidance of Archpriest Valery Danilichev. In September 2012, the Kaluga metropolitan consecrated the large bell tower, and the cathedral regained its voice. The students of the Sunday school joyfully rang the bells, which became a true celebration for them.