Flintshire (Welsh: Sir y Fflint) is a county in the north-east of Wales. It borders the Irish Sea to the north, the Dee Estuary to the north-east, the English county of Cheshire to the east, Wrexham County Borough to the south, and Denbighshire to the west. Connah's Quay is the largest town, and Flintshire County Council is based in Ewloe.

The county covers 169 square miles (440 km²), with a population of 155,000 in 2021. After Connah's Quay (16,771), the largest settlements are Flint (13,736), Buckley (16,127) and Mold (10,123). The east of the county is industrialised and contains the Deeside conurbation, which extends into Cheshire and has a population of 53,568. The adjacent coast is also home to industry, but further west has been developed for tourism, particularly at Talacre. Inland, the west of the county is sparsely populated and characterised by gentle hills, including part of the Clwydian Range and Dee Valley AONB. The county is part of the preserved county of Clwyd.

The county is named after the historic county of the same name, which was established by the Statute of Rhuddlan in 1284 and has notably different borders. The county is considered part of the Welsh Marches and formed part of the historic Earldom of Chester and Flint.

Flintshire takes its name from the historic county of Flintshire, which also formed an administrative county between 1889 until 1974 when it was abolished under the Local Government Act 1972. The principal area re-established in 1996 under the Local Government (Wales) Act 1994 does not share the same boundaries and covers a smaller area.

At the time of the Roman invasion, the area of present-day Flintshire was inhabited by the Deceangli, one of the Celtic tribes in ancient Britain, with the Cornovii to the east and the Ordovices to the west. Lead and silver mine workings are evident in the area, with several sows of lead found bearing the name 'DECEANGI' inscribed in Roman epigraphy. The Deceangli appear to have surrendered to Roman rule with little resistance. Following Roman Britain, and the emergence of various petty kingdoms, the region had been divided into the Hundred of Englefield (Welsh: Cantref Tegeingl), derived from the Latin Deceangli.

It became part of the Kingdom of Mercia by the 8th century AD, with much of the western boundary reinforced under Offa of Mercia after 752, but there is evidence that Offa's Dyke is probably a much earlier construction. By the time of the Norman conquest in 1066 it was under the control of Edwin of Tegeingl, from whose Lordship the Flintshire coat of arms is derived.

Edwin's mother is believed to have been Ethelfleda or Aldgyth, daughter of Eadwine of Mercia. At the time of the establishment of the Earldom of Chester, which succeeded the Earl of Mercia, the region formed two of the then twelve Hundreds of Cheshire of which it remained a part for several hundred years.

Flintshire today approximately resembles the boundaries of the Hundred of Atiscross as it existed at the time of the Domesday Book. Atiscross, along with the Hundred of Exestan, was transferred from the Earldom of Chester to the expanding Kingdom of Gwynedd from the west in the 13th century following numerous military campaigns. This region, as well as an exclave formed from part of the Hundred of Dudestan (known as Maelor Saesneg), later formed the main areas of Flintshire, established by the Statute of Rhuddlan in 1284 under Edward I. It was administered with the Palatinate of Chester and Flint by the Justiciar of Chester. The county was consolidated in 1536 by the Laws in Wales Acts 1535–1542 under the Tudor King Henry VIII, when it was incorporated into the Kingdom of England; it included the detached exclave of Welsh Maelor.