Member states of the Three Seas Initiative

Poland, officially the Republic of Poland, is a country in Central Europe. It extends from the Baltic Sea in the north to the Sudetes and Carpathian Mountains in the south, bordered by Lithuania and Russia to the northeast, Belarus and Ukraine to the east, Slovakia and the Czech Republic to the south, and Germany to the west. The territory is characterised by a varied landscape, diverse ecosystems, and temperate transitional climate. Poland is composed of sixteen voivodeships and is the fifth most populous member state of the European Union (EU), with over 38 million people, and the fifth largest EU country by land area, covering a combined area of 312,696 km (120,733 sq mi). The capital and largest city is Warsaw; other major cities include Kraków, Wrocław, Łódź, Poznań, and Gdańsk.

The Czech Republic, also known as Czechia, and historically known as Bohemia, is a landlocked country in Central Europe. The country is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The Czech Republic has a hilly landscape that covers an area of 78,871 square kilometers (30,452 sq mi) with a mostly temperate continental and oceanic climate. The capital and largest city is Prague; other major cities and urban areas include Brno, Ostrava, Plzeň and Liberec.

Croatia (kroh-AY-shə; Croatian: Hrvatska, ), officially the Republic of Croatia (Croatian: Republika Hrvatska listen), is a country in Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herzegovina and Montenegro to the southeast, and shares a maritime border with Italy to the west. Its capital and largest city, Zagreb, forms one of the country's primary subdivisions, with twenty counties. Other major urban centers include Split, Rijeka and Osijek. The country spans 56,594 square kilometres (21,851 square miles), and has a population of nearly 3.9 million.

Bulgaria, officially the Republic of Bulgaria, is a country in Southeast Europe. It is situated on the eastern portion of the Balkans directly south of the Danube river and west of the Black Sea. Bulgaria is bordered by Greece and Turkey to the south, Serbia and North Macedonia to the west, and Romania to the north. It covers a territory of 110,994 square kilometres (42,855 sq mi) and is the sixteenth-largest country in Europe. Sofia is the nation's capital and largest city; other major cities include Burgas, Plovdiv, and Varna.

Slovenia officially the Republic of Slovenia is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short coastline within the Adriatic Sea to the southwest, which is part of the Mediterranean sea. Slovenia is mostly mountainous and forested, covers 20,271 square kilometres (7,827 sq mi), and has a population of approximately 2.1 million. Slovene is the official language. Slovenia has a predominantly temperate continental climate, with the exception of the Slovene Littoral and the Julian Alps. Ljubljana, the capital and largest city of Slovenia, is geographically situated near the centre of the country. Other larger urban centers are Maribor, Kranj, Celje and Koper.

Slovakia, officially the Slovak Republic, is a landlocked country in Central Europe. It is bordered by Poland to the north, Ukraine to the east, Hungary to the south, Austria to the west, and the Czech Republic to the northwest. Slovakia's mostly mountainous territory spans about 49,000 square kilometres (19,000 sq mi), hosting a population exceeding 5.4 million. The capital and largest city is Bratislava, while the second largest city is Košice.

Romania is a country located at the crossroads of Central, Eastern, and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to the east, and the Black Sea to the southeast. It has a mainly continental climate, and an area of 238,397 km (92,046 sq mi) with a population of 19 million people (2023). Romania is the twelfth-largest country in Europe and the sixth-most populous member state of the European Union. Europe's second-longest river, the Danube, empties into the Danube Delta in the southeast of the country. The Carpathian Mountains cross Romania from the north to the southwest and include Moldoveanu Peak, at an altitude of 2,544 m (8,346 ft). Romania's capital and largest city is Bucharest. Other major urban centers include Cluj-Napoca, Timișoara, Iași, Constanța and Brașov.

Hungary is a landlocked country in Central Europe. Spanning 93,030 square kilometres (35,920 sq mi) of the Carpathian Basin, it is bordered by Slovakia to the north, Ukraine to the northeast, Romania to the east and southeast, Serbia to the south, Croatia and Slovenia to the southwest, and Austria to the west. Hungary has a population of 9.5 million, mostly ethnic Hungarians and a significant Romani minority. Hungarian is the official language and Budapest is the country's capital and largest city.

Austria, formally the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous city and state. Austria is bordered by Germany to the northwest, the Czech Republic to the north, Slovakia to the northeast, Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west. The landlocked country occupies an area of 83,879 km (32,386 sq mi) and has a population of around 9 million.

Lithuania (LITH-ew-AY-nee-ə; Lithuanian: Lietuva ), officially the Republic of Lithuania (Lithuanian: Lietuvos Respublika ), is a country in the Baltic region of Europe. It is one of three Baltic states and lies on the eastern shore of the Baltic Sea, bordered by Latvia to the north, Belarus to the east and south, Poland to the south, and the Russian semi-exclave of Kaliningrad Oblast to the southwest, with a maritime border with Sweden to the west. Lithuania covers an area of 65,300 km (25,200 sq mi), with a population of 2.88 million. Its capital and largest city is Vilnius. Other major cities are Kaunas, Klaipėda, Šiauliai and Panevėžys. Lithuanians belong to the ethnolinguistic group of the Balts and speak Lithuanian.