Hertfordshire (/ˈhɑːrt.fərd.ʃɪər/ ^ⓘ HART-fərd-sheer or /-ʃər/ -⁠shər; often abbreviated Herts) is a ceremonial county in the East of England and one of the home counties. It borders Bedfordshire to the north-west, Cambridgeshire to the north-east, Essex to the east, Greater London to the south and Buckinghamshire to the west. The largest settlement is Watford, and the county town is Hertford.

The county has an area of 634 square miles (1,640 km²) and had a population of 1,198,800 at the 2021 census. After Watford (131,325), the largest settlements are Hemel Hempstead (95,985), Stevenage (94,470) and the city of St Albans (75,540). For local government purposes Hertfordshire is a non-metropolitan county with ten districts beneath Hertfordshire County Council.

Elevations are higher in the north and west, reaching more than 800 feet (240 m) in the Chilterns near Tring. The county centres on the headwaters and upper valleys of the rivers Lea and the Colne; both flow south and each is accompanied by a canal. Hertfordshire's undeveloped land is mainly agricultural and much of the county is covered by the Metropolitan green belt.

Since 1903, Letchworth has served as the prototype garden city while Stevenage became the first town to expand under post-war Britain's New Towns Act 1946 (9 & 10 Geo. 6. c. 68). Services have become the largest sector of the county's economy.

The county's landmarks span many centuries, ranging from the Six Hills in Stevenage built by local inhabitants during the Roman period, to Leavesden Film Studios. The volume of intact medieval and Tudor buildings surpasses London, in places in well-preserved conservation areas, especially in St Albans, which includes remains of the Roman town of Verulamium.

In 913, Hertfordshire was the area assigned to a fortress constructed at Hertford under the rule of Edward the Elder. Hertford is derived from the Anglo-Saxon heort ford, meaning deer crossing (of a watercourse). The name Hertfordshire is first recorded in the Anglo-Saxon Chronicle in 1011. Deer feature in many county emblems. Many of the names of the current settlements date back to the Anglo-Saxon period, with many featuring standard placename suffixes attributed to the Anglo-Saxons: "ford", "ton", "den", "bourn", "ley", "stead", "ing", "lett", "wood", and "worth", are represented in this county by Hertford, Royston, Harpenden, Redbourn, Cuffley, Wheathampstead, Tring, Radlett, Borehamwood and Rickmansworth.

There is evidence of human life in Hertfordshire from the Mesolithic period. It was first farmed during the Neolithic period and permanent habitation appeared at the beginning of the Bronze Age. This was followed by tribes settling in the area during the Iron Age.

Following the Roman conquest of Britain in AD 43, the Catuvellauni tribe accepted peace and adapted to the Roman life; resulting in the development of several new towns, including Verulamium (St Albans) where in c. 293 the first recorded British martyrdom is traditionally believed to have taken place. Saint Alban, a Romano-British soldier, took the place of a Christian priest and was beheaded on Holywell Hill. His martyr's cross of a yellow saltire on a blue field is reflected in the flag and coat of arms of Hertfordshire as the yellow field to the stag or Hart representing the county. He is the Patron Saint of Hertfordshire.