I learned of this temple from a free Yale course (via iPad). It is an earlly, unusual example and perhaps the only temple in Rome with most of its original marble. And one of the most well-preserved.
As reviewers point out, it is really the Temple of Hercules Victor and is not in the Piazza Santa Maria. Google maps will find it.
Unfortunately, there is a fence around it and no signs whatsoever about it, even a name. Close by is the very well preserved Temple of Portunus.
These two temples may be the most complete ones in Rome (except for the Pantheon) and most tourists miss them or just whiz by. Easy to get to by bus from Piazza Venezia or Victor Emauelle.
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