Costa Rica is home to a rich variety of plants and animals. While the country has only about 0.25% of the world's landmass, it contains 5% of the world's biodiversity. Around 25% of the country's land area is in protected national parks and protected areas, the largest percentage of protected areas in the world (developing world average 13%, developed world average 8%).

Costa Rica has successfully managed to diminish deforestation from some of the worst rates in the world from 1973 to 1989, to almost zero by 2005. One national park, the Corcovado National Park, is internationally renowned among ecologists for its biodiversity (including big cats and tapirs) and is where visitors can expect to see an abundance of wildlife.

Corcovado is the one park in Costa Rica where all four Costa Rican monkey species can be found. These include the white-headed capuchin, the mantled howler,the endangered Geoffroy's spider monkey and the Central American squirrel monkey, found only on the Pacific coast of Costa Rica and a small part of Panama, and considered endangered until 2008, when its status was upgraded to vulnerable.

Tortuguero National Park—the name Tortuguero can be translated as "Full of Turtles"—is home to spider, howler and white-throated capuchin monkeys; the three-toed sloth and two-toed sloth; 320 species of birds; and a variety of reptiles. The park is recognized for the annual nesting of the endangered green turtle, and is the most important nesting site for the species. Giant leatherback, hawksbill, and loggerhead turtles also nest there.

The Monteverde Cloud Forest Reserve is home to about 2,000 plant species, including numerous orchids. Over 400 types of birds and more than 100 species of mammals can be found there. As a whole, around 700 species of birds have been identified in Costa Rica. The Instituto Nacional de Biodiversidad is allowed to collect royalties on any biological discoveries of medical importance. Costa Rica is a center of biological diversity for reptiles and amphibians, including the world's fastest running lizard, the spiny-tailed iguana (Ctenosaura similis).

Because Costa Rica is located between 8 and 12 degrees north of the Equator, the climate is tropical year round. However, the country has many microclimates depending on elevation, rainfall, topography, and by the geography of each particular region.

Costa Rica's seasons are defined by how much rain falls during a particular period and not to the four seasons in the Northern Hemisphere. The year can be split into two periods, the dry season known to the residents as summer, and the rainy season, known locally as winter. The "summer" or dry season goes from December to April, and "winter" or rainy season goes from May to November, which almost coincides with the Atlantic hurricane season, and during this time, it rains constantly in some regions. The location receiving the most rain is the Caribbean slopes of the Central Cordillera mountains, with an annual rainfall of over 5,000 mm (196.9 in).

Humidity is also higher on the Caribbean side than on the Pacific side. The mean annual temperature on the coastal lowlands is around 27 °C (81 °F), 20 °C (68 °F) in the main populated areas of the Central Cordilera, and below 10 °C (50 °F) on the summits of the highest mountains.