We report the discovery of a resident population of Savannah Sparrows Passerculus sandwichensis wetmorei in grassland on a high plateau of the Sierra Los Cuchumatanes in western Guatemala. We observed a nest containing a nestling, and 11 fledglings of other breeding pairs in July 2016, which represent the first confirmation of the species nesting south-east of the Isthmus of Tehuantepec, c.700 km from the nearest known breeding locality in central Mexico. We provide the first description of juvenile plumage for this subspecies. Furthermore, we describe the song of P. sandwichensis wetmorei for the first time, which is similar in structure and signal characteristics to that of northern populations. Based on transects, we estimate a population density of 0.7–4.3 adults / ha in the study area, in the Parque Regional Municipal Todos Santos Cuchumatán, which is similar to published data from the USA and Canada. This is the first documentation of P. sandwichensis wetmorei since the type specimens were collected more than 100 years ago.