Isaiah did. More interestingly though, we can ask, how much of it did he write? The question comes up because of how different chapters 1-39 seem to be compared to chapters 40-66. For example, Assyria was a major power in Isaiah’s day, and it gets mentioned often in chapters 1-39, but we don’t find any mentions of it in chapters 40-66. Why not? The answer to the question of how much Isaiah wrote is a little tricky. As far as I know, the main view among Evangelicals is that he wrote of all it.

One reason to conclude this is that the New Testament references seem to understand Isaiah to be the author of even the supposed later sections (chapters 40-66). For example, Luke 3:4-6 references Isaiah 40:3-5. However, it could also just be viewed as reference to the book itself. Luke 3:4 says “written in the book of the words of Isaiah.”

Some defenders of single authorship have asserted that a multiple authorship view is actually an attempt to undermine the predictive prophecy found in Isaiah such as the references to Cyrus in chapters 44 and 45. This is an unnecessary critique however, because proponents of multiple authorship would still uphold the predictive prophecy of the suffering servant in chapter 53.

A third option is available. It may have been mostly written by Isaiah with a little editorial activity sprinkled in later times which might help explain some of the anomalies. Whatever your view is, as long as you still think it’s inspired by God, then that’s good enough for me.