Miserentissimus Redemptor (English: "Most Merciful Redeemer") is the title of an encyclical written by Pope Pius XI, promulgated on May 8, 1928, on the theology of reparation to the Sacred Heart.

In the opening section of the encyclical, the Pope stated that Jesus Christ had "manifested Himself" to Margaret Mary Alacoque and had "promised her that all those who rendered this honor to His Heart would be endowed with an abundance of heavenly graces".

[1]: Section 1 Referencing Pope Leo XIII's encyclical Annum sacrum, Pius stated, "For as in olden time when mankind came forth from Noe's ark, God set His "bow in the clouds" (Genesis ix, 13), shining as the sign of a friendly covenant; so in the most turbulent times of a more recent age, ... then the most benign Jesus showed his own most Sacred Heart to the nations lifted up as a standard of peace and charity portending no doubtful victory in the combat.

Pius XI said that devotion to the Sacred Heart provided a succinct summary of the entire spiritual life.

[1] Pope Francis, in his encyclical letter on the Sacred Heart (2024) refers to Pius' teaching as an aspect of Christian spirituality which it is "fitting to recover".