Eastern Marmara is bordered by Istanbul Province, Southern Marmara, Central Anatolia, and Western Black Sea Region to northwest, southwest, south/southeast, and east respectively. To the north, it occupies part of Turkish Black Sea coast, while to the west, the Gulf of İzmit, part of the Sea of Marmara, makes a long indentation into the region.
Eastern Marmara more or less corresponds to Bithynia of ancient times.
Being the cradle of Ottoman Emirate, later to become Ottoman Empire, this region is dotted by historical towns and villages maintaining traditional Ottoman architecture, if a bit weary. North of the region is the Black Sea coast with some resort villages largely missed by travellers, while on the coasts of Lake Sapanca and Lake İznik, two of the biggest lakes of the vicinity, are a number of summer resorts and beaches mostly frequented by Turkish mid-class families who prefer a milder (and tamer) alternative to Aegean and Mediterranean coasts.
Most of the region is mountainous, or at least hilly, and covered with verdant forests.
A railway line coming in from Istanbul, lies west-east along the northern shore of the Gulf of İzmit and southern shore of Lake Sapanca and passes through İzmit and Adapazarı, where it turns southwards and continues along the bottom of impressive rocky and wooded Sakarya Valley towards Eskişehir. This is part of the Istanbul–Ankara railway line, so has the most frequent intervals of passenger trains on any route in the country, at least once every two hours 8:30AM-midnight, with the section of the line between Istanbul and Adapazarı having even more frequent departures—which start earlier as well, around 6:30AM.