From the perspective of Catholicism are there any non-Catholic saints?

Are there any non-Catholic saints?

The short answer seems to be in the affirmative!

Saint Seraphim of Sarov (1754-1833) seems to fit the bill here. He is an Eastern Orthodox Saint and has been glorified in their tradition. He is in many Catholic lists of saints and Pope John Paul II addressed him as a saint!

Thus there are a few non-Catholic saints recognized by Rome.

Saint Seraphim of Sarov (Russian: Серафим Саровский) (1 August [O.S. 19 July] 1754 (or 1759) – 14 January [O.S. 2 January] 1833), born Prokhor Moshnin (Прохор Мошнин), is one of the most renowned Russian monks and mystics in the Eastern Orthodox Church. He is generally considered the greatest of the 19th-century startsy (elders). Seraphim extended the monastic teachings of contemplation, theoria and self-denial to the layperson. He taught that the purpose of the Christian life was to acquire the Holy Spirit. Perhaps his most popular quotation amongst Orthodox believers is "Acquire a peaceful spirit, and thousands around you will be saved."

Seraphim was glorified (canonized) by the Russian Orthodox Church in 1903. His biographer Nicholas Motovilov was one of his "spiritual children". Pope John Paul II referred to him as a saint. (Wikipedia).

There are many Catholic websites online that include Seraphim of Sarov as being a saint.

Here is what Catholic Saints Info has to say about St Seraphim of Sarov:

Son of a builder, he had a middle-class upbringing. Monk at Sarov in 1777, taking the name Seraphim. Studious as a boy, he was able to apply himself there as the monks of Sarov spent much of the day studying Scripture and the early Church writings. Severely ill and bed-ridden from 1780 to 1783, Seraphim continued his studies, and received repeated apparitions of the Virgin Mary. Ordained in 1793, he celebrated Mass daily, which was unusual at the time.

In 1794 he became a hermit in the forest near the Sarov monastery. In 1804 he was severely beaten by thieves, and left for dead; he dragged himself to the monastery, spent five months in recovery, and spent the rest of his life stooped and requiring a cane to walk. He lived for a while atop a pillar, then in a walled up cell. Offered the abbacy of Sarov in 1807, but declined, and lived the next three years without speaking.

In 1810 his health had deteriorated to the point that he could no longer live in the woods. He returned to the Sarov abbey, and lived as a hermit within its walls. In 1832 he received a vision from the Virgin Mary that told him to return to the world and give others the benefit of his wisdom. He attracted followers and students, became known as a healer, and was called by the honourific starets, Russian for spiritual teacher. Many of his teachings have been reprinted in the West, and Pope John Paul II referred to him in the book Crossing the Threshold of Hope (as a saint).

Our Sunday Visitor's Encyclopedia of Saints lists the feast day of St Seraphim of Sarov as being celebrated in the Catholic Church on January 2.

Furthermore, in Pope John Paul II's apostolic letter Tertio Millennio Adveniente (1994), he made a very strong allusion to Martyrs of Christ of various faiths: Catholics, Orthodox, Anglicans and Protestants:

37."The Church of the first millennium was born of the blood of the martyrs: "Sanguis martyrum - s*m*n christianorum".(21) The historical events linked to the figure of Constantine the Great could never have ensured the development of the Church as it occurred during the first millennium if it had not been for the seeds sown by the martyrs and the heritage of sanctity which marked the first Christian generations. At the end of the second millennium, the Church had once again become a Church of martyrs. The persecutions of believers — priests, religious and laity — has caused a great sowing of martyrdom in different parts of the world. The witness to Christ borne even to the shedding of blood has become a common inheritance of Catholics, Orthodox, Anglicans and Protestants, as Pope Paul VI pointed out in his Homily for the Canonization of the Ugandan Martyrs.(22)

This witness must not be forgotten. The Church of the first centuries, although facing considerable organizational difficulties, took care to write down in special martyrologies the witness of the martyrs. These martyrologies have been constantly updated through the centuries, and the register of the saints and the blessed bears the names not only of those who have shed their blood for Christ but also of teachers of the faith, missionaries, confessors, bishops, priests, virgins, married couples, widows and children.

In our own century the martyrs have returned, many of them nameless, "unknown soldiers" as it were of God's great cause. As far as possible, their witness should not be lost to the Church. As was recommended in the Consistory, the local Churches should do everything possible to ensure that the memory of those who have suffered martyrdom should be safeguarded, gathering the necessary documentation. This gesture cannot fail to have an ecumenical character and expression. Perhaps the most convincing form of ecumenism is the ecumenism of the saints and of the martyrs. The communio sanctorum speaks louder than the things which divide us. The martyrologium of the first centuries was the basis of the veneration of the Saints. By proclaiming and venerating the holiness of her sons and daughters, the Church gave supreme honour to God himself; in the martyrs she venerated Christ, who was at the origin of their martyrdom and of their holiness. In later times there developed the practice of canonization, a practice which still continues in the Catholic Church and in the Orthodox Churches. In recent years the number of canonizations and beatifications has increased. These show the vitality of the local Churches, which are much more numerous today than in the first centuries and in the first millennium. The greatest homage which all the Churches can give to Christ on the threshold of the third millennium will be to manifest the Redeemer's all-powerful presence through the fruits of faith, hope and charity present in men and women of many different tongues and races who have followed Christ in the various forms of the Christian vocation.

It will be the task of the Apostolic See, in preparation for the Year 2000, to update the martyrologies for the universal Church, paying careful attention to the holiness of those who in our own time lived fully by the truth of Christ. In particular, there is a need to foster the recognition of the heroic virtues of men and women who have lived their Christian vocation in marriage. Precisely because we are convinced of the abundant fruits of holiness in the married state, we need to find the most appropriate means for discerning them and proposing them to the whole Church as a model and encouragement for other Christian spouses."

Rome may one day start canonizations for others Christian denominational martyrs as Martyrs for Christ!

Let us not forget that outside the normal process of canonization, the Pope remains free to pronounce an equipollent canonization at his discretion.