William Kelly (May 1821 – 27 March 1906) was a prominent Irish member of the Plymouth Brethren, amongst whom he was a prolific writer.

As editor of the latter he was brought into correspondence with Henry Alford, Robert Scott the lexicographer, Principal Edwards and William Sanday of Oxford, among others.

He identified himself whole-heartedly with the body of doctrine developed by John Nelson Darby, whose right-hand man he was for many years, until he severed his connection and formed a faction which bore his name.

Within Darby's lifetime, Kelly was known to outsiders for his lectures on the Pentateuch, the Gospel of Matthew, the Revelation of John, the Church of God, and the New Testament doctrine of the Holy Spirit, besides notes on Romans.

After 1890 he issued In the Beginning, commended by Archbishop Benson and expositions of the prophecies of Isaiah, of the Gospel of John, of the epistle to the Hebrews, of the epistles of John; a volume of 600 pages on God's Inspiration of the Scriptures and his last words on Christ's Coming Again, in which he vindicates the originality of Darby's teaching in regard to the Secret Rapture.