Kocaeli, one of the 6 provinces of Turkey neighboring two seas, has a strong relationship with the Marmara Sea in the south and the Black Sea in the north. The fact that the 30-degree east meridian passes through Köseköy is one of the factors that point to the importance of Kocaeli's location.

The Samanlı Mountains, starting from the west of the Sakarya River and extending to Bozburun in the north of Pamukova and İznik Lake, dominate the depression area covering İzmit, Sapanca, and Adapazarı. The highest point of these mountains at 1602 meters is home to Kartepe Ski Center. Dikmen Mountain, Naldöken Mountain, Naz Mountain, and Cenedag are the other mountains of the province.

Tectonic depressions such as İzmit Bay and Sapanca Lake, basins such as the Black Sea, and changes in the sea surface have formed the present form of the Kocaeli Peninsula. Touching two seas, the city has many small stream valleys. The rivers of the city pouring into the Black Sea are longer. While Riva Stream and Ağva Stream, which originate in Kocaeli, flow into the Black Sea in Istanbul, the main stream flowing into the Black Sea within the borders of Kocaeli is Sarısu Stream. Originating from the Samanlı Mountains, Kirazdere meets the Sea of Marmara in the Gulf of Izmit.

Located in the western part of Kocaeli, 7 km of Sapanca Lake is included in Kocaeli and borders Uzuntarla, Maşukiye, and Eşme. The artificial lakes behind the Kirazdere Dam and Yuvacık Dam, which supply water to Izmit, are the city's important water resources. Gebze, Kandıra, Kartepe, and Körfez counties are home to 23 caves in the province.

The climate of Kocaeli, which provides a transition between the Mediterranean climate and the Black Sea climate, is temperate in the Gulf of Izmit and the Black Sea coasts compared to the interior parts, and a harsher course in the mountainous parts. There is a temperature difference between the shores of the two seas in the same season. On the shores of the Samanlı Mountains facing the Gulf, the climate is similar to the Black Sea climate. Annual precipitation, which exceeds 1000 mm on the Black Sea coast, decreases as you go south.

Vegetation in Kocaeli shows characteristics of the Black Sea in the north and the Mediterranean in the south, depending on the climate. The vegetation, which has many endemic species, shows itself mostly with the diversity of the Marmara Region. The rate of endemic plants reaches 3.39% and 1477 different plant species grow throughout the province. Plants such as “Kilyos Button”, “Istanbul Nazendesi”, “Hoşkangal” and “Sıktarlakuşu”, which are grown only in our country, are also seen in Kocaeli. The plant that Kocaeli lands brought to the world is “Keltepe Crocus”.