Arroyo Piray Guazú is a tributary of the río Paraná located in the province of Misiones, Argentina. Its name derives from Guaraní and translates roughly as "river of the fish" (or "large fish water"). The stream originates at Cerro Alegría in the northern sector of the Sierra de Misiones and flows to its mouth approximately two kilometers upstream of the town of Puerto Piray, in the Departamento Montecarlo.

Two paddling routes run along the arroyo. The first and longer segment begins at the confluence with Arroyo Anta and ends at the Balneario Municipal in the city of Eldorado, covering roughly 30 kilometers and best paddled when the arroyo is running at good volume. The second, shorter segment of approximately 16 kilometers starts at the Balneario Municipal and finishes at the curved bridge on Ruta Nacional 12 — locally known as the "puente banana" for its distinctive curved form — which was built in early 1969 and links the Departamentos of Montecarlo and Eldorado. Paddlers have noted that this lower stretch can run very shallow at times of low water, with the potential for hull contact and minor damage to kayaks.

The Balneario Municipal Piray Guazú in Eldorado serves as a practical access and take-out point and is a well-used public facility, particularly busy during the summer months (December through February in the Southern Hemisphere). Operating hours at the balneario are 7:00 to 20:00. Several campsites are located in the vicinity, including Camping La Colina, Camping Piray Jungle, and Cerne Turismo Rural, providing overnight options for paddlers.

The arroyo passes through dense Misionero jungle, and the corridor is noted for its biodiversity. The neighboring Arroyo Piray Miní, which shares similar landscape characteristics, supports species such as Yacutinga (Pipile jacutinga), Cutirí duck (Amazonetta brasiliensis), Muscovy duck (Cairina moschata), and capybara, suggesting comparable wildlife along the Piray Guazú corridor. Guided kayak excursions on the arroyo are offered commercially out of Eldorado, typically in small groups of up to four people.