The Pleistos (Greek: Πλείστος, Ancient Greek: Πλεῖστος, Latin: Pleistus) is a river in central Greece. It drains the Pleistos valley, named after it, a relatively recent rift valley north of the Gulf of Corinth, and parallel to it. They have the same geologic causes. Being situated in karst topography, much of the river runs or seeps through underground channels. The surface stream is intermittent. However, the limestone riverbed reflecting the light gives the appearance of a stream of water.

The semi-arid valley floor, too inaccessible for urban development, is eminently suitable for dendriculture. Extensive olive groves, nicknamed the "sea of olives," have been in place since prehistoric times. The floor is flanked by precipitous elevations, notably a scarp on the north side. The primary access road to the valley runs on the side of the north scarp throughout its entire length.

Near the lower valley the road intersects the site of ancient Delphi. Oracular temples have existed there since Mycenaean times. The spring system at Delphi drops into the Pleistos. The lower valley was a seat of Mycenaean power, with capital at Krisa. The Gulf of Corinth was then named the Gulf of Krisa, but in early classical times the states of southern Greece combined to remove Krisa from its predominance in the region.

The true sources of the water in the upper Pleistos are the numerous springs that exude from the base of the north scarp, and waterfalls that pour over it. The scarp is after all the flank of Parnassos. Some ground water must be seeping from it continually as though it were a sieve. This water in all the cracks breaks out rocks by freezing and thawing, while in the soil of the scree it contributes to the fluidity, making landslides more likely. Rockfalls and mudslides are common along the valley, making protection by steel mesh fences a necessity in places, and closing some features of Delphi to the public. Earthquakes, which render the soil momentarily into a fluid, are all the more devastating. Buildings destroyed by them are likely to fall down the scree into the Pleistos.

The river begins from sources on the side of Mount Parnassos below the town of Arachova, Boeotia, at approximately 38°28′34.8″N 22°35′26.8″E / 38.476333°N 22.590778°E / 38.476333; 22.590778. The elevation is approximately 700 metres (2,300 ft).

The river flows west through a deep valley, between the mountains Parnassos and Kirphe, passing south of Delphi, through the Delphic Landscape and the Krisaean plain and reaches the Gulf of Itea, a bay of the Corinthian Gulf, near Kirra. The water of the Castalian Spring system flows into the Pleistos. The river enters the Gulf of Cornth undramatically through a culvert of the coastal road on the east side of Cirra. A stream a few inches deep leaves the culvert to cross a small delta, geologically of antique origin. This stream is alternately labelled the Pleistos or the Cirra River. On the other side of the road it comes from a wetland passing by St. John's Church. The wetland originates further north from a ravine in Mount Cirphis, but it does not receive any waters above ground from the flow of Pleistos.

A few yards to the west of the culvert is another culvert under the same road, but without a delta. Its water comes from an apparent ditch beside the Itea Peripheral Road. Northward this ditch leaves the road and becomes a controlled channel through the olive groves. Along it are private farmhouses and footbridges. The channel is continuous with the stream in the Pleistos Valley. The visible bed is usually empty. If Pleistos means "full" as some say it does, the use must be an irony.

Apparently the hydrologic channels were altered in the management of the groves. They cover the entire non-urban areas of the valley system and are called locally "the sea of olives." The stream with the braided delta must represent the more ancient stream, the original Pleistos. During the reconfiguration of the hydrology, the Pleistos was disconnected from its wetlands and forced to irrigate olive trees. The climate is semi-arid. The wetlands then became the Cirra. A similar nomenclature discrepancy exists on the west side of the valley. The Skitsa River erodes the Amfissa Valley and then courses in a straight, controlled channel to the gulf at Itea, irrigating the west side of the valley. The sources say that it also was formerly named the Plistos, implying that the same Plistos river drained both valleys before different channels were dredged.