lowermost of four main divisions in the Silurian (Silurian Period) System, representing those rocks deposited worldwide during the Llandovery Epoch (443.7–428.2 million years ago). The name of the series is derived from the type district, around the town of Llandovery in Dyfed, southern Wales, where about 1,200 metres (about 4,000 feet) of fossiliferous shales (shale), sandstones (sandstone), and gray mudstones (mudstone) occur.

The base of the Llandovery Series is coincident with the base of the Silurian System. As formally defined in 1985 under the authority of the International Commission on Stratigraphy (ICS), the global stratotype section and point (GSSP) for this boundary is established 1.6 metres (5.2 feet) above the base of the Birkhill Shale Formation on the north side of the Linn Branch stream at Dob's Linn, near Moffat in southern Scotland. The boundary point, or “golden spike,” is marked by the base of the graptolite biozone of Parakidograptus acuminatus fixed by the first occurrence of that species together with the graptolite Akidograptus ascensus. Other key reference districts for the Llandovery Series are located in the Oslo region of southern Norway and on Anticosti Island in Quebec, Can. Shelly facies in all three districts are characterized by brachiopods ( lamp shells) belonging to the genera Stricklandia, Pentamerus, and Eocoelia. The top of the Llandovery Series is defined by the base of the overlying Wenlock Series. It is underlain by the Hirnantian Stage of the Upper Ordovician (Ordovician Period) Series. The Llandovery Series is subdivided into three worldwide stages: the Rhuddanian, Aeronian, and Telychian stages.