The site where the church of San Giovanni di Sinis is located corresponds to a sector of the Phoenician-Punic necropolis of the ancient city of Tharros. The persistence of the cemetery use of the area in Christian times is witnessed by various finds, including an epigraphic funeral table with a circular shape. Recent excavations have found the remains of an older Christian cult building.
The church of San Giovanni di Sinis has a strong impact thanks to the contrast with the marine landscape, the proximity of the spectacular Capo San Marco, where the ruins of the ancient city of Tharros insist, its polished forms and the dome that completes the curvilinear rhythms of the architectural volumes.
The building is the result of the longitudinal trench transformation of a Byzantine church with an inscribed cross plan, dating back to the 6th-7th century, of which the domed body and the transverse arms remain, with mullioned windows opened in the proto-Romanesque period (11th century).
The observation from the outside makes it possible to identify the various buildings: a central cube raised above the side volumes that reveal the transept surmounted by a dome and closed to the E by the apse.
Inside the church you can see the traces of the cushioning of a barrel vault originally set at a lower level than the current one, the result of reconstruction. Also part of the expansion phase are the apse, characterized by the outflow receding the tax line, and the blind arches leaning against the sides of the aisles.
In the oldest part of the church, we can see with what skill the Byzantine builders built the dome, connecting the square space delimited at the top by the pillars with the circular structure of the dome. The connection is made possible by the use of plumes, a solution that is both daring and elegant, common to other areas under the influence of Constantinople.

History of studies
The church was mentioned as early as the 19th century by Vittorio Angius. In 1953 Raffaello Delogu proposed ascribing it to the 5th century, returning an inscribed cross structure, expanded in the 8th-9th century, replacing the western area with a tri-naved room. The interpretation was accepted by Roberto Coroneo (1993), who, however, delayed the longitudinal transformation to the eleventh century.

Bibliography
V. Angius, “Cabras”, in Goffredo Casalis, Historical-Statistical-Commercial Geographical Dictionary of the States by H.M. the King of Sardinia, III, Turin, G. Maspero, 1836, pp. 6-18;
D. Scano, History of Art in Sardinia from the 11th to the 14th Century, Cagliari-Sassari, Montorsi, 1907, pp. 19-29;
R. Delogu, The Architecture of the Middle Ages in Sardinia, Rome, The State Library, 1953, pp. 13-14; R. Coroneo, Romanesque Architecture from the Mid Thousand to the Middle Ages in Sardinia, Rome, The State Library, 1953, pp. 13-14; R. Coroneo, Romanesque Architecture from the Mid Thousand to the Middle Ages in Sardinia, Rome, The State Library, 1953, pp. 13-14; R. Coroneo, Romanesque Architecture from the Mid Thousand to the Middle Ages in Sardinia, Rome, The State Library, 1953, pp. 13-14; R. Coroneo, Romanesque Architecture from the Mid Thousand to the Middle Ages early '300, Nuoro, Ilisso
, 1993, sheet 6;
P.G. Spanu, Byzantine Sardinia between the 6th and 7th centuries, Oristano, S'Alvure, 1998, pp. 66-72; R. Coroneo-M.
Coppola, Byzantine Cruciform Churches of Sardinia, Cagliari, 1999, pp. 37-39; M.G. Messina-D.
Mureddu, “New archaeological elements from Saint John of Sinis”, in Insulae Christi. Primitive Christianity in Sardinia, Corsica and the Balearic Islands, Oristano, S'Alvure, 2002, pp. 239-244; R. Coroneo-R. Serra, Pre-Romanesque and Romanesque Sardinia, series “Italian Artistic Heritage”, Milan, Jaca Book, 2004, pp. 45-51;
R. Coroneo, Romanesque Churches of Sardinia. Tourist-cultural itineraries, Cagliari, AV, 2005, pp. 79.

How to get there
You leave the SS 131 at the height of Siamaggiore to take the SP 12 towards Zeddiani, continuing to Riola Sardo on the SP 66. After a few kilometers, turn south on SP 62 and take SP 7 in the direction of San Salvatore, where you take SP 6 for the last stretch of the route to Tharros, to the sea and to the church of San Giovanni di Sinis.