Likely the oldest of the valley's temples, the Temple of Heracles dates to the final years of the 6th century BC, dedicated to Heracles (Hercules to the Romans), one of the most venerated figures in ancient Akragas. Its style is noticeably more archaic than its neighbours: taller, more widely spaced columns with wide, flattish capitals, features that place it a generation or more before temples like Concordia and Juno.
According to Cicero, the temple once held a bronze statue of Heracles so beloved that its lips and chin were worn smooth from the kisses and caresses of devotees. Gaius Verres, the notoriously corrupt Roman governor of Sicily who looted temples across the island, is said to have had his eye on it, though there's no record he actually got away with this one.
An earthquake brought the temple down long before modern times. The row of eight standing columns you see today is a 1923 reconstruction, and an unusually personal one: it was funded and overseen by Sir Alexander Hardcastle, a retired English army captain who fell in love with Agrigento, moved into the nearby Villa Aurea, and spent his entire personal fortune excavating and rebuilding this corner of the valley.
The Romans later added small cult rooms at the rear of the building, where excavators found a large statue of the healing god Asclepius, now in the Archaeological Museum. The temple sits near Villa Aurea, about a kilometre west of the Temple of Concordia, on the quieter end of the site.