Doubts persist about the identity and origins of the colossal temple on the Quirinal Hill in Rome, of which only small fragments remain. The present article confirms its identity as a temple of Serapis, but ascribes it to Hadrian and Antoninus Pius rather than the Severans, as most scholars since Hülsen have done. The Severan phase, it argues, was limited to a few possible restorations of the temple proper, a new surrounding portico, and the grand opus latericium staircase ascending the hill to the temple. It traces the journey of a series of Proconnesian columns from the incipient Pantheon porch, where Hadrian’s engineers failed to find a way to erect them; to the new Serapeum on the Quirinal, designed to accommodate the colossal order that had failed the Pantheon; and finally, after a partial collapse of the Serapeum porch, to the Basilica of Maxentius, whence Paul V transferred a single column shaft to the Piazza S. Maria Maggiore. Special attention is given to antiquarian illustrations of the remains, especially those of Palladio, whose Pantheon-like plan of the Serapeum porch is vindicated; and to the statuary that is attested in the vicinity of the temple.