Eminent Scottish architect Robert Adam considered this temple one of the most beautiful European monuments. Rectangular in its floor plan the temple served to celebrate the Jupiter's cult. It lies on an elevated podium, with a six column porch in front of it. Embossed images on the portal, as well as the barrel coffered vault influenced the early Renaissance architecture of Andrija Alessi and Nikola Firentinac in Trogir.

The transformation into a Baptistery happened in the Late Antiquity era, with a construction of a crypt under the building dedicated to St Thomas. In the former temple, at the beginning of the 13th century, a Baptismal Font was made of the altar screen pluteus (11th century), originally seated in the cathedral. One of the posts shows a figure of a Croatian king (Petar Krešimir IV or Zvonimir), making it the earliest presentation of a European king on a Medieval stone sculpture.

The Baptistery today is dominated by a Secession sculpture of St John the Baptist, whose name the temple carried after the transformation, this was the work of Ivan Meštrović, while in front of it one of several completely or partly preserved granite sphinxes was placed that Diocletian brought from Egypt. The Baptistery is open for visitors, with an entrance fee.