An endemic frog species which has not been sighted for three decades and was declared extinct, has made a shocking reappearance in Costa Rica.

The Heredia robber frog, whose scientific name is Craugastor escoces, had been declared extinct by the International Union for Conservation of Nature in 2004 and had not been observed since 1986.

But Costa Rican biologists Gilbert Alvarado and Randall Jimenez reported on Tuesday spotting the frog in the Juan Castro Blanco National Park in the Alajuela province.

"In Costa Rica there are eight of them and in the 1990s, all of those living above an altitude of 1000 metres disappeared or nothing more was known of them," Alvarado told reporters.

"This one (the Heredia robber frog) was declared extinct in 2004 but now we're going to bring it back."

The discovery of the frog, which has more than 30 species of its kind from Mexico to Panama, occurred in September 2016 while collecting a sample of amphibians.

The discovery occurred in a ravine at an altitude of 1820 metres, which makes researchers think there must be a robber frog population surviving there.

Official data indicates that in this Central American country there are more than 200 species of frogs and toads.