The Alegre River (Portuguese: Rio Alegre) is a river in the state of Mato Grosso, Brazil.

The Alegre River rises in the 120,092 hectares (296,750 acres) Serra de Santa Bárbara State Park, created in 1997.

It flows in a generally northwest direction, receiving the Barbado River on its left, then flows north until it joins the Guaporé River from the left.

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