The Royal Welch Fusiliers (Welsh: Ffiwsilwyr Brenhinol Cymreig) was a line infantry regiment of the British Army and part of the Prince of Wales' Division, founded in 1689 shortly after the Glorious Revolution. In 1702, it was designated a fusilier regiment and became The Welch Regiment of Fusiliers; the prefix "Royal" was added in 1713, then confirmed in 1714 when George I named it The Prince of Wales's Own Royal Regiment of Welsh Fusiliers.

After the 1751 reforms that standardised the naming and numbering of regiments, it became the 23rd Foot (Royal Welsh Fuzileers).

It retained the archaic spelling of Welch, instead of Welsh, and Fuzileers for Fusiliers; these were engraved on swords carried by regimental officers during the Napoleonic Wars. After the 1881 Childers Reforms, its official title was The Royal Welsh Fusiliers, but "Welch" continued to be used informally until restored in 1920 by Army Order No.56.

It should not be confused with the Welch Regiment, a different unit (formed in 1881 from the 41st and 69th) which recruited in South and West, rather than North Wales, and became part of the Royal Regiment of Wales or RRW in 1969.One of the few regiments to retain its original title, in March 2006 the Royal Welch Fusiliers was amalgamated with the RRW and became 1st Battalion, Royal Welsh, with RRW as the 2nd Battalion.