The Eocene sedimentary units in the Kocaeli and Armutlu peninsulas and Iznik-Geyve region (northwest Turkey) consist of marine siliciclastic and carbonate deposits, locally intercalated with volcaniclastic rocks and tuffs (Middle Eocene Magmatic Rocks-MEMR). The record of larger benthic fauna (LBF) and pelagic microfossils, such as planktonic foraminifera and calcareous nannoplankton, combined with lithological description, indicate a wide spectrum of depositional environments ranging from shallow-to deep-marine. These deposits contain in situ and/or allochthonous assemblages of orthophragminids, nummulitids, subordinate rotaliids, and sparse alveolinids. The in situ assemblages (mainly orthophragminids) point to several episodes of shallow marine depositional conditions during a) early Ypresian (SBZ 5-7 and 7-9), b) late Ypresian (SBZ 10-11), c) early Lutetian (SBZ 13), and d) middle Lutetian (SBZ 14-15). Upper Ypresian (Cuisian) deposits with rich LBF accumulations are the most ubiquitous, recognized across a wide region from the Thrace Basin to Central Anatolia. Lower Lutetian deposits, on the other hand, occur in a limited area only near the town of Sile, on the coast of the Black Sea. Orthophragminids immediately below the first volcaniclastic beds of the MEMR constrain the initiation of the volcanism to the latest Ypresian or around the Ypresian/Lutetian boundary. The LBF assemblages from the clastic intercalations of the MEMR are referred to SBZ 14-15 and indicate that the marine realm in the Armutlu and Kocaeli peninsulas persisted until middle Lutetian. Since then, continental conditions have prevailed in the region, while a marine realm has developed in the Thrace Basin during the late Middle to Late Eocene and Oligocene. The deep-marine Eocene deposits with pelagic microfossils and allochthonous LBF are mostly confined to the Ypresian. The allochthonous orthophragminids in these deep-marine deposits are also presented here to give a broader picture of their occurrence in NW Turkey.