Marmara Ereğlisi (Turkish: [ˈmaɾmaɾa æɾeːlisi]; Greek: Ηράκλεια), also spelled Marmaraereğlisi, is a municipality and district of Tekirdağ Province, Turkey.

The town, originally a Samian colony, was founded as Perinthos (Greek: Πέρινθος), in English usually known by its Latinized form as Perinthus.

It was built amphitheatre-like on the hillside of a cape extending into the Sea of Marmara, close to where the modern town stands.

However, Heraclea was recognized in the Notitia Episcopatuum of Pseudo-Epiphanius as having five suffragan sees: Panium, Callipolis, Chersonesus in Europa, Coela, and Rhaedestus.

There is a long coastline and the sea is clean enough for swimming, (not true of much of the Marmara) and the coast on either side of Ereğli is lined with hotels and compounds of holiday properties serving people from Istanbul, who come to relax in the summer sunshine.

Ereğli is only an hour's drive from Istanbul and on a summer Sunday evening the road is a solid queue of returning weekenders.

The town of Ereğli and its nearby villages are used by these weekenders and summer residents for fast food, grocery shopping, internet cafes and other amenities.

The town itself is a mixture of large modern blocks and old country houses, both types mostly having been built without proper planning or architectural design.