A solemn vow is a certain vow ("a deliberate and free promise made to God about a possible and better good") taken by an individual after completion of the novitiate in a religious order. It is solemn insofar as the Church recognizes it as such, and is unique in that no human authority within the church can relieve a person from it.

Any vow in Catholic religious life other than a solemn vow is a simple vow. Even a vow accepted by a legitimate superior in the name of the Church (the definition of a "public vow") is a simple vow if the Church has not granted it recognition as a solemn vow. In canon law a vow is public (concerning the Church itself directly) only if a legitimate superior accepts it in the name of the Church; all other vows, no matter how much publicity is given to them, are classified as private vows (concerning directly only those who make them). The vow taken at profession as a member of any religious institute is a public vow, but in recent centuries can be either solemn or simple.

There is disagreement among theologians as to whether the distinction between solemn and simple vows is primarily a matter of ecclesiastical administration or theological in nature; the latter position follows the opinion of Thomas Aquinas, holding that a solemn vow is, regardless of its approbation by the Church, a more strict, perfect and complete consecration to God.

Aquinas held that the only vows that could be considered solemn were those made by receiving the holy orders as a member of the Catholic Hierarchy, or by the religious profession of the rule as a member of a Catholic religious order.

As a unique^{[citation needed]} exception to this traditional dichotomy, the Benedictine abbots could be consecrated bishops by an analogue apostolic authority (like another bishop, an archbishop, or the pope). This practice was contemplated by the canonical law since the Middle Age, as it is testified by the later life of Peter Cellensis. Since the 18th century, consecrators and episcopal lineage were extended to the Benedictine monks-bishops.

Aquinas, in support of his view, cited the fact that these two vows alone were considered to make the celebration of marriage invalid. He argued that a man who promised, either to a human being or to God (thus making a vow), to marry a certain woman was bound by that promise or vow, but if he broke it and married a different woman, the subsequent marriage was nonetheless considered valid. Similarly, if he made a vow to enter a particular religious institute or become a priest, but instead entered a different institute or decided to marry, the religious profession or the marriage, despite being a violation of his vow, was still considered valid. Once he had received holy orders or made a religious profession, however, any marriage he contracted was considered null and void.

Solemn vows were originally considered indissoluble. Not even the Pope could dispense from them. If for a just cause a religious was expelled, the vow of celibacy remained unchanged and so rendered invalid any attempt at marriage, the vow of obedience obliged in relation, generally, to the bishop rather than to the religious superior, and the vow of poverty was modified to meet the new situation but the expelled religious "could not, for example, will any goods to another; and goods which came to him reverted at his death to his institute or to the Holy See".

In medieval times religious vows taken by members of religious institutes approved by the Holy See were regarded both as solemn and public. This was declared by Pope Boniface VIII (1235–1303).