比提尼亚（古希腊语：Βιθύνια）小亚细亚西北部的一个古老地区、王国及罗马行省，与普罗庞提斯海、色雷斯、博斯普鲁斯海峡及黑海（古称攸克辛海）相邻。

有多个城市座落于普罗庞提斯海（即现今的马摩拉海）的肥沃海岸上：尼科米底亚（Nicomedia），卡尔西顿（Chalcedon）、基俄斯（Cius）及阿帕米亚（Apamea）。比提尼亚亦包括尼西亚（Nicaea，因尼西亚信经而闻名，今称为伊兹尼克）。

根据斯特拉博所述，比提尼亚东面与桑加瑞斯河（Sangarius，即现今撒卡亚河Sakarya）接壤，但更普遍受接纳的是边境伸延至Parthenius河，使比提尼亚与帕夫拉戈尼亚（Paphlagonia）分隔，因而让玛利安杜尼亚人（Mariandyni）栖息于此。在西与西南面有林达库斯河（Rhyndacus）使与密细亚人分隔；南面与弗里吉亚及加拉太相邻。

Bithynia (/bᵻˈθɪniə/; Greek Βιθυνία Bithynia) was an ancient region, kingdom and Roman province in the northwest of Asia Minor, adjoining the Propontis, the Thracian Bosporus and the Euxine Sea. It bordered on Mysia to the south-west, Paphlagonia to the north-east along the Pontic coast, and Phrygia to the south-east towards the interior of Asia Minor. Bithynia was an independent kingdom from 4th century BC. Its capital Nicomedia was rebuilt on the site of ancient Astacus in 264 BC by Nicomedes I of Bithynia. Bithynia fell to the Roman Republic in 74 BC, and became united with the Pontus region as the province of Bithynia et Pontus, in the 7th century incorporated into the Byzantine Opsikion theme. It became a border region to the Seljuk Empire in the 13th century, and was eventually conquered by the Ottoman Turks in the 1330s.