Traditionally, the benthic foraminifera Hyalinea balthica (Schroter) has been considered a species with high intraspecific variability, possibly related to climatic changes, and, therefore, a potential indicator of paleoclimate in the Mediterranean Sea. However, a stable and characteristic morphology very different from the syntype of the species can be observed in specimens from the Sea of Marmara. By identification and documentation of this morphology and a subsequent comparison to specimens similar to the type species, clear differences between the two groups have been established. Hyalinea marmarica n. sp. is proposed as an example of possible environment-related speciation in benthic foraminifera. It differs from H. balthica in having less numerous chambers in the last whorl (maximum of eight), depressed sutures at least in the last four chambers, a moderate increase in chamber size as added, a subacute profile and accessory apertures on the peripheral margin. Speciation could have occurred between 14 and 12 ka. At this time, increasingly efficient connections were progressively established between the Sea of Marmara and the Eastern Mediterranean Sea, allowing Hyalinea balthica to migrate into the Sea of Marmara. Here this species might have modified its morphology in response to the extreme environmental conditions.