The Convention on Access to Information, Public Participation in Decision-making and Access to Justice in Environmental Matters - also known as the Aarhus Convention - "protects every person's right to live in an environment adequate to his or her health and well-being", according to the UN.
It gives citizens the right to participate in decision-making in environmental matters and "acknowledges that we owe an obligation to future generations", its website says.
The convention and its Protocol on Pollutant Release and Transfer Registers are "the only legally binding global instruments on environmental democracy", it says.
Guinea-Bissau, a country of around two million, acceded to the convention on April 4, bringing the total number of parties to 47.
It is the first country "outside the pan-European region" to do so, Olga Algayerova, the executive secretary of the United Nations Economic Commission for Europe (UNECE), said.
"Guinea-Bissau hopes to take advantage of the instruments of the Convention to fight climate change (and) promote its biodiversity... allowing public participation in decision-making as well as access to justice when their rights to the environment are violated", Environment Minister Viriato Luis Soares Cassama said.
The West African state is "one of the most vulnerable countries to climate change," UNECE said.
It is threatened by flooding and increased salinization in coastal regions, which affect agriculture and can lead to drinking water shortages, among other adverse affects.
Mining and construction activities also threaten the country's protected areas.
In January, police blocked activists from protesting plans to destroy a park in the heart of the capital Bissau as part of a construction project.
In 2021, Guinea-Bissau acceded to the UN Water Convention, becoming the fourth African country to do so.