Welsh republicanism (Welsh: Gweriniaetholdeb Gymreig) or republicanism in Wales (Welsh: Gweriniaetholdeb yng Nghymru) is the political ideology that advocates for Wales to be governed by a republican system, as opposed to a monarchy, such as the current British monarchy.

Typically, this ideology is suggested as an element of forming an independent Wales, but can also be considered as part of reforming the political system of the United Kingdom, which could include introducing an elected official as head of state. It is not synonymous with Welsh independence, but one possible form of government, as Wales before English rule had native monarchies, and most of the prominent independence organisations do not explicitly support republicanism. While the Wales Green Party supports a republic and would support independence in a referendum.

During the Middle Ages and since the Roman Empire left the island of Britain, Wales had many monarchical states most notably the kingdoms of Gwynedd, Powys, and Deuheubarth. The most significant Welsh royal house was the House of Gwynedd which started in c. 401 and its cadet branches, Aberffraw, Dinefwr, and Mathrafal.

The legendary King Arthur is embedded in Welsh tradition and national stories through his battles for Celtic independence from the encroachment of Anglo-Saxon forces. Many Welsh leaders referenced King Arthur's messianic return to save the Welsh people and to lead rebellions against England.^{[citation needed]}

Welsh unification fluctuated over time between different kingdoms, including the Anglo-Norman Kingdom of England which held land in southern Wales from the late 11th century. Llywelyn the Great of Gwynedd formally united Wales at the Council of Aberdyfi in 1216 under the Principality of Wales with the title Prince of Wales. The title of Prince of Wales was used not as one subservient to a king (as it is now), but to invoke the Roman Imperial title of princeps.^{[citation needed]}

Some native Welsh nobles would continue to hold a few positions of power or status in Wales after English conquest. Owain Glyndŵr as a member of the House of Mathrafal shortly reasserted Wales and an independent principality of Wales 1400–1409. The Welsh Tudors of Penmynydd were a prominent royal house which supported Glyndŵr, and would then become English monarchs after Henry VII won the War of the Roses.

In the 13th century, the last Prince of Wales, Llywelyn the Last was forced into an agreement by Edward I of England that saw Llywelyn withdraw his powers to Gwynedd only. In 1282 whilst attempting to gather support in Cilmeri near Builth Wells, Llywelyn was killed by one of Edward's soldiers. Llywelyn's brother, Dafydd ap Gruffydd briefly led a force in Wales, but was captured and later hanged, drawn and quartered by Edward, thus ending Welsh independence.

Since conquest, there have been Welsh rebellions against English rule. The last, and the most significant revolt was the Glyndŵr Rising of 1400–1415, which briefly restored independence. Owain Glyndŵr held the first Welsh parliament (Senedd) in Machynlleth in 1404 where he was proclaimed Prince of Wales and a second parliament in 1405 in Harlech. Following the eventual defeat of the Glyndŵr rebellion and a brief period of independence, it wasn't until 1999 that a Welsh legislative body was re-established as the National Assembly of Wales which was renamed Senedd Cymru/Welsh Parliament in 2020.