The Rincon de la Vieja volcano, in the northwestern region of the country, burst into a huge column of smoke on Monday. The volcano is located in a national park in Guanacaste province some 200 kilometres from the capital, San Jose.
The outburst began in the early hours and lasted for nearly three minutes.
The thick smoke column was almost two kilometres (1.2 miles) high.
Ash from the column poured across the surrounding region but there were no casualties from it.
Local residents were not immediately evacuated.
Maarten de Moor, a specialist at the Volcanological and Seismological Observatory of Costa Rica, said it was “a pretty energetic eruption”.
The volcano has frequent eruptions, with 1,400 registered last year.
However, Mr de Moor pointed out that Monday’s eruption “could be the largest since the 1990s.”
The National Commission for Risk Prevention and Emergency Management was analysing the situation.