On which island was John when he received the visions of Revelation?

Answer: Patmos

John writes that he was on the island of Patmos "because of the word of God and my testimony about Jesus."

“I, John, your brother and partner in the tribulation and kingdom and perseverance that are in Jesus, was on the island of Patmos because of the word of God and my testimony about Jesus.”— Revelation 1:9 (BSB)