Cyrenaica (/ˌsaɪrəˈneɪ.ɪkəˌˌsɪr-/ SY-rə-NAY-ik-ə-,-SIRR-ə) or Kyrenaika (Arabic: برقة, romanized: Barqah, Koine Greek: Κυρηναϊκή [ἐπαρχία], romanized: Kūrēnaïkḗ [eparkhíā], after the city of Cyrene), is the eastern region of Libya. Cyrenaica includes all of the eastern part of Libya between the 16th and 25th meridians east, including the Kufra District. The coastal region, also known as Pentapolis ("Five Cities") in antiquity, was part of the Roman province of Crete and Cyrenaica, later divided into Libya Pentapolis and Libya Sicca. During the Islamic period, the area came to be known as Barqa, after the city of Barca.

Cyrenaica became an Italian colony in 1911. After the 1934 formation of Italian Libya, the Cyrenaica province was designated as one of the three primary provinces of the country. During World War II, it fell under British military and civil administration from 1943 until 1951, and finally in the Kingdom of Libya from 1951 until 1963. The region that used to be Cyrenaica officially until 1963 has formed several shabiyat, the administrative divisions of Libya, since 1995. The 2011 Libyan Civil War started in Cyrenaica, which came largely under the control of the National Transitional Council (headquartered in Benghazi) for most of the war. In 2012, a body known as the Cyrenaica Transitional Council unilaterally declared Cyrenaica to be an autonomous region of Libya.

Geologically, Cyrenaica rests on a mass of Miocene limestone that tilts up steeply from the Mediterranean Sea and falls inland with a gradual descent to sea level again.

This mass is divided into two blocks. The Jebel Akhdar extends parallel to the coast from the Gulf of Sidra to the Gulf of Bomba and reaches an elevation of 882 meters. There is no continuous coastal plain, the longest strip running from the recess of Gulf of Sidra past Benghazi to Tolmeita. Thereafter, except for deltaic patches at Susa and Derna, the shore is all precipitous. A steep escarpment separates the coastal plain from a relatively level plateau, known as the Marj Plain, which lies at about 300 meters elevation. Above the Marj Plain lies a dissected plateau at about 700 meters elevation, which contains the highest peaks in the range.

The Jebel Akhdar and its adjacent coast are part of the Mediterranean woodlands and forests ecoregion and have a Mediterranean climate of hot, dry summers and relatively mild and rainy winters. The plant communities of this portion of Cyrenaica include forest, woodland, maquis, garrigue, steppe and oak savanna. Garrigue shrublands occupy the non-agricultural portions coastal plain and coastal escarpments, with Sarcopoterium spinosum, along with Asphodelus ramosus and Artemisia herba-alba, as the predominant species. Small areas of maquis are found on north-facing slopes near the sea, becoming more extensive on the lower plateau. Juniperus phoenicea, Pistacia lentiscus, Quercus coccifera and Ceratonia siliqua are common tree and large shrub species in the maquis. The upper plateau includes areas of garrigue, two maquis communities, one dominated by Pistacia lentiscus and the other a mixed maquis in which the endemic Arbutus pavarii is prominent, and forests of Cupressus sempervirens, Juniperus phoenicea, Olea europaea, Quercus coccifera, Ceratonia siliqua, and Pinus halepensis.

Areas of red soil are found on the Marj Plain, which has borne abundant crops of wheat and barley from ancient times to the present day. Plenty of springs issue on the highlands. Wild olive trees are abundant, and large areas of oak savanna provide pasture to the flocks and herds of the local Bedouins. Historically large areas of range were covered in forest. The forested area of the Jebel Akhdar has been shrinking in recent decades. A 1996 report to the UN Food and Agriculture Organization estimated that the forested area was reduced to 320,000 hectares from 500,000 hectares, mostly cleared to grow crops. The Green Mountain Conservation and Development Authority estimates that the forested area decreased from 500,000 hectares in 1976 to 180,000 hectares in 2007.

The southward slopes of the Jebel Akhdar are occupied by the Mediterranean dry woodlands and steppe, a transitional ecoregion lying between the Mediterranean climate regions of North Africa and the hyper-arid Sahara.

The lower Jebel el-Akabah lies to the south and east of the Jebel Akhdar. The two highlands are separated by a depression. This eastern region, known in ancient times as Marmarica, is much drier than the Jebel Akhdar and here the Sahara extends to the coast. Historically, salt-collecting and sponge fishing were more important than agriculture. Bomba and Tobruk have good harbors.