Unam sanctam is a papal bull that was issued by Pope Boniface VIII on 18 November 1302. It laid down dogmatic propositions on the unity of the Catholic Church, the necessity of belonging to it for eternal salvation, the position of the Pope as supreme head of the Church and the duty thence arising of submission to the Pope in order to belong to the Church and thus to attain salvation. The Pope further emphasized the higher position of the spiritual in comparison with the secular order. The historian Brian Tierney calls it "probably the most famous" document on church and state in medieval Europe. The original document is lost, but a version of the text can be found in the registers of Boniface VIII in the Vatican Archives. The bull was the definitive statement of the late medieval theory of hierocracy, which argued for the temporal as well as spiritual supremacy of the pope.

The bull was promulgated during an ongoing dispute between Boniface VIII and King Philip IV of France (Philip the Fair). Philip had levied taxes on the French clergy of half their annual income. On 5 February 1296, Boniface responded with the papal bull Clericis laicos that forbade clerics, without authority from the Holy See, to pay taxes to temporal rulers, and threatened excommunication on rulers who demanded such payments.

King Edward I of England defended his own taxing powers by putting defiant clergy under outlawry, a Roman law concept withdrawing their protection under the English common law, and confiscated the temporal properties of bishops who refused his levies. As Edward was demanding an amount well above the tenth offered by the clergy, Archbishop of Canterbury Robert Winchelsey left it to every individual clergyman to pay as he saw fit.

In August 1296 King Philip imposed an embargo forbidding export of horses, arms, gold, and silver, effectively keeping the French clergy from sending taxes to Rome and blocking a main source of papal revenue. Philip also banished from France papal agents raising funds for a new crusade.

In September 1296 the pope sent a protest to Philip headed Ineffabilis Amor that declared that he would rather suffer death than surrender any of the rightful prerogatives of the Church. While threatening a papal alliance with England and Germany, the pope soothingly explained that his claims were not intended against the customary feudal dues, and that reasonable taxation of Church revenue would be permitted. To assist their king against the Anglo-Flemish alliance, the French bishops asked permission to make contributions for the defense of the kingdom. In February 1297, Boniface issued Romana mater ecclesia, declaring that when the clergy consented to make payments and delay could cause grave danger, papal permission could be dispensed, and ratified the French payments in the encyclical Corum illo fatemur. While insisting that Church consent was required for subsidies to the state, he recognised that the clergy in each country must evaluate such claims. In July 1297, Boniface, further beset by an uprising in Rome by the Colonna family, again moderated his assertions in Clericis laicos. The bull Etsi de statu allowed lay authorities to declare emergencies to tax clerical property.

The Jubilee year of 1300 filled Rome with fervent masses of pilgrims who supplied the lack of French gold in the treasury. The following year, Philip's ministers overstepped their bounds. In the Albigensian Crusade, the suppression of the Cathar heresy had brought much of Languedoc under French royal control, but in the farthest south, heretics still survived, and Bernard Saisset, Bishop of Pamiers in Foix, was recalcitrant and insolent with king. Philip's ministry decided to make an example of the bishop, who was brought before the royal court on 24 October 1301. The chancellor, Pierre Flotte, charged him with high treason, and put the bishop in the keeping of his metropolitan, the archbishop of Narbonne. Before Saisset could be tried, the royal ministry needed the Pope to strip the bishop of his office and protections, a "canonical degradation". Instead, in December 1301 Boniface ordered the bishop to Rome to justify himself before his pope rather than his king. In the bull Ausculta Fili ("Give ear, my son"), he scolded Phillip: "Let no one persuade you that you have no superior or that you are not subject to the head of the ecclesiastical hierarchy, for he is a fool who so thinks." At the same time, Boniface sent out the general bull Salvator mundi strongly reiterating Clericis laicos.

With his customary tactlessness, Boniface then summoned the French bishops to Rome to reform their national Church matters. Philip forbade Saisset or any bishop to attend, and organized a counterassembly of his own in Paris in April 1302. Nobles, burgesses and clergy met to denounce the Pope and pass around a crude forgery, Deum Time ("Fear God"), in which Boniface supposedly claimed feudal suzerainty over France, an "unheard-of assertion". Boniface denied the document and its claims, but reminded Phillip that previous popes had deposed three French kings.

This was the atmosphere in which Unam sanctam was promulgated weeks later. Reading of the "two swords" (the spiritual and temporal powers), one of Philip's ministers is alleged to have remarked, "My master's sword is steel; the Pope's is made of words". As Matthew Edward Harris writes, "The overall impression gained is that the papacy was described in increasingly exalted terms as the thirteenth century progressed, although this development was neither disjunctive nor uniform, and was often in response to conflict, such as against Frederick II and Philip the Fair".