The Welsh Marches (Welsh: Y Mers) is an imprecisely defined area along the border between England and Wales in the United Kingdom. The precise meaning of the term has varied at different periods.

The English term Welsh March (in Medieval Latin Marchia Walliae) was originally used in the Middle Ages to denote the marches between England and the Principality of Wales, in which Marcher lords had specific rights, exercised to some extent independently of the king of England. In modern usage, "the Marches" is often used to describe those English counties which lie along the border with Wales, particularly Shropshire and Herefordshire, and sometimes adjoining areas of Wales. However, at one time the Marches included all of the historic counties of Cheshire, Shropshire, Herefordshire, Worcestershire and Gloucestershire.

The term March is from the 13th-century Middle English marche ("border region, frontier"). The term was borrowed from Old French marche ("limit, boundary"), itself borrowed from a Frankish term derived from Proto-Germanic *markō ("border, area"). The term is a doublet of English mark, and is cognate with German Mark ("boundary"). Cognates are found in the English toponyms "Mercia" and "Mersey", and in continental place-names containing mark, such as "Denmark".

The term is distantly related to the verb march, both ultimately derived from Proto-Indo-European *mereg-, "edge" or "boundary".

After the decline and fall of the Roman Empire which occupied southern Britain until about AD 410, the area which is now Wales comprised a number of separate Romano-British kingdoms, including Powys in the east. Over the next few centuries, the Angles, Saxons and others gradually conquered and settled in eastern and southern Britain. The kingdom of Mercia, under Penda, became established around Lichfield, and initially established strong alliances with the Welsh kings.

However, his successors sought to expand Mercia further westwards into what is now Cheshire, Shropshire and Herefordshire. As the power of Mercia grew, a string of garrisoned market towns such as Shrewsbury and Hereford defined the borderlands as much as Offa's Dyke, a stronger and longer boundary earthwork erected by order of Offa of Mercia between AD 757 and 796. The Dyke still exists, and can best be seen at Knighton, close to the modern border between England and Wales. Campaigns and raids from Powys led, possibly around about AD 820, to the building of Wat's Dyke, a boundary earthwork extending from the Severn valley near Oswestry to the Dee estuary.

In the centuries which followed, Offa's Dyke largely remained the frontier between the Welsh and English. Æthelstan, often seen as the first king of a united England, summoned the British kings to a meeting at Hereford in AD 926, and according to William of Malmesbury laid down the boundary between Wales and England, particularly the disputed southern stretch where he specified that the River Wye should form the boundary.

By the mid-eleventh century, Wales was united under Gruffydd ap Llywelyn of Gwynedd, until his death in 1063.^{[citation needed]}