The Temple of Hadrian, originally misidentified as the Basilica of Neptune, was dedicated to the deified Emperor Hadrian by his successor, Antoninus Pius, in the mid-2nd century CE. While only part of the structure remains today, it once stood as a grand architectural statement in the heart of Rome. Excavations and scholarly analysis have provided valuable insights into its construction techniques and stylistic influences, allowing historians to reconstruct its significance within Imperial Rome.

The Temple of Deified Hadrian was situated in the Campus Martius, near significant landmarks such as the Solarium Augusti and the Column of Marcus Aurelius, which was built later. Ancient records, including the Notitia, place it in Regio IX, in proximity to the Baths of Alexander Severus and the Baths of Agrippa. It is also believed that the Temple of Matidia, dedicated to Hadrian’s mother-in-law, was located just to the west. Some scholars suggest that the temple may have featured grand entrances on both ends, though no definitive architectural evidence supports this theory. The temple was enclosed within a large colonnaded precinct, sections of which were uncovered in early excavations led by Rodolfo Lanciani.

Excavations of the site commenced in 1878, with more recent investigations in the cellars of buildings surrounding Piazza di Pietra revealing the outline of a monumental enclosure wall. This enclosure featured a large curved exedra at its rear, suggesting an impressive architectural design. Additionally, thorough excavation of the colonnade’s front section, located behind the railing, uncovered the original ground level of the temple precinct—approximately 5 meters below the present square. These findings also exposed the high podium’s flank, which was originally clad in white marble, harmonizing with the columns above.

circa 145 CE

The Temple of Hadrian was an octastyle structure, meaning it featured eight columns across its façade. It was elevated on a high podium, a typical feature of Roman temple design, and followed the peripteral style, with a surrounding colonnade. The entrance likely consisted of a staircase covering the eastern end, leading to a deep pronaos with three bays. During Hadrian’s reign, there was a brief resurgence of peripteral temples in Rome, as seen in the Temple of Venus and Roma, which shares several architectural similarities with the Temple of Hadrian.

Both temples exhibit a two-stepped architrave and cornice profile, though instead of traditional modillions, the cornice was supported by plain consoles. A sima decorated with palmettes and lion heads, attributed by Frank Sear to the same group of architects, further emphasizes the connection between these structures. However, a key difference is the frieze design—while the Temple of Venus and Roma follows a standard format, the Temple of Hadrian's frieze was pulvinated (convex) rather than flat. Drawings and fragmentary remains suggest that its architrave was adorned with garlands in swags, while the frieze featured an elaborate acanthus candelabrum motif intertwined with reversing S-spirals.

Inside the Borsa building, remnants of the non-apsidal naos (inner chamber) can still be observed. This space was originally covered by a coffered barrel vault, supported by columns that framed battle trophies. The column bases were decorated with reliefs depicting personifications of Roman provinces, emphasizing Hadrian’s diplomatic and administrative approach in contrast to the military conquests of his predecessor, Trajan. Many of these reliefs are now housed in the National Roman Museum and the Capitoline Museums.

As one of Antoninus Pius’ earlier construction projects, the temple reflects stylistic influences from late Hadrianic architecture, including a departure from the traditional Corinthian Order. Its design also exhibits Asiatic and Pergamene elements, characterized by rich ornamentation and an eclectic mix of influences. The square-shaped cella was lined with engaged columns, and its barrel-vaulted ceiling—constructed of concrete—featured coffered designs with traces of marble paneling, sections of which survive inside the Borsa building. Nearly every surface of the temple was lavishly decorated, incorporating a blend of Asiatic motifs, engaged orders, and pilasters.

Surrounding the temple was a large square arcade supported by giallo antico columns. This colonnade opened onto the Via Lata (modern Via del Corso) through a triumphal arch. Historical records have alternately identified this arch as the "Arch of Antoninus", the "Arch of Claudius", and the "Arch of the Tosetti" (named after the family residing in Piazza Sciarra, a location later demolished during road expansions). By the 18th century, despite its ruin, this arch still gave its name to the Via dell’Archetto.

The temple's materials further highlight its architectural significance. The Corinthian columns, measuring 1.44 meters in diameter and 14.8 meters in height, were made from Proconnesian marble, a distinctive stone with grey and white horizontal banding sourced from northwestern Turkey. This material did not appear in Rome until the late Hadrianic period but was widely utilized under the Severan dynasty. The cella walls, situated behind the colonnade, were constructed of peperino tufa, which was faced with white marble to match the columns. The blocks of peperino were left rough, likely intended for a marble revetment that was never completed. Some of the fluted colonnade columns were also made of giallo antico, a prized yellow Numidian marble from Tunisia, frequently used for columns, flooring, and decorative veneers.

circa 145 CE

Only one side of the Temple of Deified Hadrian remains intact today, with both ends and the opposite side having long since disappeared. What survives includes eleven fluted Corinthian columns, complete with their bases and capitals, as well as a section of the cella wall, which was later incorporated into a 19th-century palazzo that still houses the Rome Borsa.

The lower portion of the entablature, originally adorned with intricate carvings, has been preserved, while the upper sections are known from 16th-century drawings. In modern restorations, much of the entablature has been reconstructed using stucco, but the cornice restoration is inconsistent, with three distinct versions—only the central one resembling the original design.

Beneath the temple’s front steps, remnants of vaulting indicate that the structure originally faced eastward, toward the Corso. It is believed to have had a front colonnade of eight columns and thirteen along each side. Surviving clamps suggest that pilasters were once attached to the colonnade, further emphasizing the temple’s grand architectural design.