The book of Revelation is traditionally said to be written by John.  The first verse in the Book of Revelation does not say exactly who that particular John is, but it is naturally assumed to be John the apostle since it was the apostles that were given the authority by God to write Scripture.

“The Revelation of Jesus Christ, which God gave Him to show to His bond-servants, the things which must soon take place; and He sent and communicated it by His angel to His bond-servant John,” (Rev. 1:1).

Also, early church historians attributed the book of Revelation to John the apostle as well.

Justin Martyr [Dialogue with Trypho, p. 308] (a.d. 139–161) quotes from the Apocalypse, as John the apostle’s work, the prophecy of the millennium of the saints, to be followed by the general resurrection and judgment. This testimony of Justin is referred to also by Eusebius [Ecclesiastical History, 4.18]. Justin Martyr, in the early part of the second century, held his controversy with Trypho, a learned Jew, at Ephesus, where John had been living thirty or thirty-five years before: he says that “the Revelation had been given to John, one of the twelve apostles of Christ.”¹Jamieson, R., Fausset, A. R., Fausset, A. R., Brown, D., & Brown, D. (1997). A commentary, critical and explanatory, on the Old and New Testaments. On spine: Critical and explanatory commentary. (Re 1:1). Oak Harbor, WA: Logos Research Systems, Inc.