The cathedral was consecrated in 1484 to host the relic of the "Sacra Tovaglia," a miracle that occurred not far away, in Gruaro. The events date back to 1294. During the washing of sacred vestments coming from Valvasone, blood stains appeared on the fabric of a cloth, believed to have come from a consecrated host forgotten inside it. The building began construction in 1449 in late Romanesque style, was furnished during the following century, and renovated at the end of the 1800s in neo-Gothic style, with numerous wooden elements replaced by marble and a new facade. The church houses a perfectly functioning organ from the 1500s, the only surviving one still existing in Italy from the famous Venetian Renaissance tradition. The organ boasts gilded friezes and painted panels by Pomponio Amalteo and Giovanni Antonio de’ Sacchis known as "il Pordenone," among the most renowned artists of the time.

The main religious building in Valvasone is dedicated to the Holy Body of Christ, a title that summarizes and explains much of its historical and artistic events.

In fact, according to tradition, the cathedral was built following an event that occurred in Gruaro (a locality now in the province of Venice), along the Versiola canal, where in 1294 (but more likely in 1394) a pious woman, intent on washing a cloth from the nearby church of San Giusto, noticed a consecrated host, negligently forgotten in the linen by a doubtful officiant, from which blood was flowing, indelibly marking the fabric.

Documents attest that at the beginning of the 15th century the sacred cloth had been transported to Valvasone and placed on a special altar of the ancient parish church dedicated to Santa Maria delle Grazie and San Giovanni, which, after the construction of the cathedral, underwent a decline that led to its slow but inexorable decay, concluded with the definitive demolition in 1866.

On March 28, 1454, Pope Nicholas V decreed that the precious cloth be entrusted to the Valvason family (who in the meantime had ceded the castle of Gruaro to the abbot of Sesto, in exchange for the villas of San Lorenzo and Orcenico Superiore), on the condition that they build a new church to house it, to be dedicated, precisely, to the Holy Body of Christ.

The Roman decree overlapped with the decision, made in 1449 in the 13th-century church of San Giacomo (located in the current former Post Office), under the auspices of Count Giacomo Giorgio di Valvason (the coats of arms of the counts, patrons of the cathedral, are clearly visible in various places in the church, making it appear almost like a palatine chapel), to build a new religious building in which to also preserve the relic, within the second circle of walls, in an area of comital property, replacing the inadequate and decentralized parish church.

The construction work of the new center of religious life in Valvasone did not proceed particularly quickly, perhaps due to the presence of a still usable parish church, and only in 1466 was the roof covered; while the sacred relic was transferred to the new and final home around 1479, when the church was certainly officiated, although the completion of the building is recorded around 1484, when on September 8, on the day of the birth of Mary, the solemn consecration took place.

From an architectural point of view, the building presents itself with an extremely simple and austere appearance, which follows the typical schemes of the Franciscan order (in turn derived from the Cistercian one), perhaps also due to practical needs related to the chosen site.

The building structure we admire today is essentially the same as the original 15th-century one: a large rectangular hall, oriented as usual towards the east (on a site that is lower than the surrounding square), with a single nave, with a roof with exposed trusses, while pointed arches frame the rectangular-plan presbytery and the two chapels that flank it (the one on the left less deep to make room for the mighty bell tower, which still houses a bell cast in 1733, with the metal of a specimen from two centuries earlier).

Originally, the facade was meant to faithfully reflect the austerity of the interior, featuring a gabled roof and a single entrance in the center, topped by a large oculus, with possibly two smaller ones on either side, symbolically recreating the Holy Trinity.

The presbytery, on the other hand, was illuminated by two tall pointed arch windows, which were bricked up in the 19th century and replaced with a rose window. Recently, the two openings have been restored, with the consequent removal of the 19th-century addition, returning this part of the building to its original structure; among other things, this has allowed the highlighting of the remains of a phytomorphic fresco decoration on the back wall.

The current appearance of the facade and much of the temple is due to a series of radical building interventions carried out between 1889 and the early 20th century (with the intervention by Luigi Paolo Leonardon from San Vito).

Moreover, the new late 19th-century facade – emphasized by the demolition of the tower that stood in front of it – radically changed the function of the northern side, which, due to the particular urban layout of the area, had essentially played the role of the main facade (similarly to what happens in the cathedral of Spilimbergo), as can still be seen today from the elaborate terracotta frieze and the presence of decorative frescoes with geometric and figurative motifs (as indeed all the facades of the buildings surrounding the sacred building were certainly frescoed), among which stand out the remains of some busts of saints, inserted in the space in the hanging arches under the cornice.

The interior of the cathedral, despite its over five hundred years of existence, does not deviate too much from the original intention to create an environment of mystical simplicity, in which the attention of the faithful was to be directed exclusively towards the sacred Eucharistic relic.

This intent is made even more evident by the recent modifications (of 2004) of the apse area, culminating with the repositioning of the 17th-century altar placed under the sacred arch, thus restoring the ancient division of space.

The main altar, a work from the second half of the 17th century, reproduces the architecture of a small temple, enriched with polychrome marbles, in whose tabernacle since 1793 the sacred cloth is kept. Above it, a large wooden Crucifix has been placed, attributed to Pomponio Amalteo or, rather, to his workshop, dated around 1556-1557 and created in a deliberately late style, so much so as to recall examples from the previous century.

In the 16th century, the main altar was adorned with a now-lost altarpiece depicting the Savior, while the sacred linen was kept in a small gilded copper niche.

Following the recommendations expressed in 1584 by the apostolic visitor Cesare de Nores, bishop of Parenzo, who followed counter-reformation directives, the main altar was equipped with a tabernacle in which to place the Holy Eucharist at the center of the choir and no longer, as also happened in Valvasone, in a simple niche.

During the 17th century, the two side chapels, the one on the left dedicated to Saints James the Greater and Christopher and the other to Saint Catherine of Alexandria, also underwent radical reforms.

Along the side walls are placed two other altars: on the left, the one dedicated to Saint Nicholas bishop and on the opposite side, that of the Holy Cross.

The first is in neo-Gothic style, made during the late 19th-century works that also involved the facade, and replaces a 1678 altar by the Meduno stonecutters Giuseppe and Daniele Ciotta. It houses an altarpiece by the Venetian painter Matteo Luigi Canonici, paid for in 1791, depicting Saint Nicholas in prayer before the Virgin with the Child.

Instead, the altar dedicated to the Holy Cross – which in 1576 Countess Giulia di Valvasone endowed with a legacy of 1000 ducats – was executed in 1705 by Francesco Caribolo, undergoing subsequent heavy modifications at the beginning of the 20th century, and hosts an altarpiece by the painter Anzolo di Portogruaro from 1605, depicting Saint Helena and the discovery of the Cross.