What was the last letter added to the alphabet?

The last letter added to the English alphabet is J. Although the letter J has existed in some form since ancient times, it wasn't until the late 15th century that it became differentiated from the letter I and represented a distinct phonetic sound. Prior to that, the letters I and J were often used interchangeably in medieval writings. As the English language evolved, the necessity for a distinct letter that represented the "j" sound led to the formal adoption of J as a separate letter. This historical context highlights why J is considered the last letter added to the English alphabet.