The book does not name an author
Jonah opens by identifying the prophet who received God's command, but it never says, “Jonah wrote this book.” The story is told mostly in the third person, describing what Jonah did, feared, prayed, and said. That leaves the human author unnamed (Jonah 1:1).
This isn't unusual in Scripture. Several biblical books do not identify their writer within the text. So the most careful answer is simple: the author of Jonah is unknown. We can be confident about the message God gave through the book without claiming more than the text tells us.
Now the word of the Lord came unto Jonah the son of Amittai, saying,
Was it Jonah himself?
Many Jewish and Christian readers have traditionally understood Jonah to be the author, or at least the source behind the account. Jonah was a real prophet associated with Gath-hepher in Israel during the reign of Jeroboam II. In 2 Kings, he is named as the son of Amittai, just as he is in Jonah (2 Kings 14:25; Jonah 1:1).
That connection makes Jonah's involvement very plausible. Who else could so readily report his prayer from inside the fish, his anger outside Nineveh, and God's searching questions to him? Jonah 2 records a deeply personal prayer, while Jonah 4 exposes the prophet's reluctance and resentment with striking honesty (Jonah 2:1-2; Jonah 4:1-4).
Then Jonah prayed unto the Lord his God out of the fish's belly, And said, I cried by reason of mine affliction unto the Lord, and he heard me; out of the belly of hell cried I, and thou heardest my voice.
Why do some people see a later writer?
Others suggest that a later prophet, scribe, or faithful compiler wrote the book using Jonah's own testimony. They point to the third-person storytelling and to occasional explanations that may help readers unfamiliar with Nineveh. The book itself does not settle that question.
This view does not require treating Jonah as fiction or dismissing his ministry. Jesus spoke of Jonah's time in the great fish and of Nineveh's response to his preaching as significant examples for his own generation (Matthew 12:40-41). Jesus also linked Jonah's experience to his own death and resurrection (Matthew 12:40).
The message matters most
Jonah is more than a record of an unusual rescue at sea. It shows the Lord pursuing a runaway prophet, hearing his prayer, warning a violent city, and showing compassion when its people turn from evil. When Nineveh responded, God withheld the judgement he had announced (Jonah 3:5; Jonah 3:10).
The closing scene is especially revealing. God challenges Jonah's narrow concern for a plant while reminding him of the many people and animals in Nineveh (Jonah 4:10-11). Whether Jonah personally wrote every line or someone preserved his account, the book directs us to the same truth: God's mercy reaches further than our instincts often do. Read Jonah slowly, and notice not only what God asks of Nineveh, but what he is patiently teaching Jonah.
Then said the Lord, Thou hast had pity on the gourd, for the which thou hast not laboured, neither madest it grow; which came up in a night, and perished in a night: And should not I spare Nineveh, that great city, wherein are more than sixscore thousand persons that cannot discern between their right hand and their left hand; and also much cattle?