The Royal Welch Fusiliers was an infantry regiment of the British Army, part of the Prince of Wales' Division. It was founded in 1689 to oppose James II and to take part in the imminent war with France. The regiment was numbered as the 23rd Regiment of Foot, though it was one of the first regiments to be granted the honour of a fusilier title and so was known as The Welsh Regiment of Fusiliers from 1702. The "Royal" accolade was earned fighting in the War of the Spanish Succession in 1713.

It was one of the oldest regiments in the regular army, hence the archaic spelling of the word Welch instead of Welsh. In the Boer War and throughout the First World War, the army officially called the regiment "The Royal Welsh Fusiliers" but the archaic "Welch" was officially restored to the regiment's title in 1920 under Army Order No.56. During those decades, the regiment itself unofficially used the "Welch" form. The regiment was amalgamated with the Royal Regiment of Wales (RRW) on 1 March 2006, to become 1st Battalion, The Royal Welsh (RRW becoming the 2nd Bn).

The regiment primarily recruited from North Wales. It should not be confused with the Welch Regiment, which recruited from South and West Wales.

Soldiers of this regiment were distinguishable by the unique feature of the "flash", consisting of five overlapping black silk ribbons (seven inches long for soldiers and nine inches long for officers) on the back of the uniform jacket at neck level.[18] This is a legacy of the days when it was normal for soldiers to wear pigtails. In 1808, this practice was discontinued but when the order was issued the RWF were serving in Nova Scotia and had not received the instruction when the regiment departed to join an expedition to the West Indies.[19] In 1834 the officers of the 23rd Foot were finally granted permission by William IV to wear this non-regulation item as a distinction on the full dress uniform as "a peculiarity whereby to mark the dress of that distinguished regiment.