Temple of Janus, Ancient temple ruins in Forum Holitorium, Rome, Italy

The Temple of Janus is the northernmost of three contiguous Republican temples on the west side of the Forum Holitorium, in the area of the church of San Nicola in Carcere. Its remains are seven tuff columns with their architrave built into the right flank of the church, plus two free-standing columns near the Theatre of Marcellus. The temple measured about 26 by 15 metres and had an Ionic hexastyle pronaos.

Gaius Duilius built the temple in the 3rd century BC after the Roman victory at Mylae in 260 BC, probably over an earlier shrine. The Senate was allegedly barred from meeting there, and its dedication day was the Portunalia of 17 August. Augustus began a restoration that Tiberius completed on 18 October AD 17, adding a statue of Janus by Scopas or Praxiteles.

The temple's annual festival was held on the Portunalia, 17 August, the day of its first dedication; after Tiberius completed the restoration on 18 October AD 17, the festival moved to October. The Senate was reportedly barred from meeting here, since a decree sending the Fabii to besiege Veii had been made in a temple of Janus.

The seven tuff columns and architrave are built into the right flank of San Nicola in Carcere, so the best view is from outside along the church's right side; two further columns stand free near the Theatre of Marcellus. The church interior also exposes the temple's tuff columns below the present floor.

The temple's ruins are not a separate monument: seven tuff columns with their architrave are built into the right flank of the church of San Nicola in Carcere, and two more stand free near the Theatre of Marcellus. The Senate was reportedly barred from meeting here, since its decree sending the Fabii to besiege Veii had been passed in a temple of Janus.