Jeremiah was the prophet behind the book
The opening of the book identifies Jeremiah as its central prophetic voice. He was the son of Hilkiah, a priest from Anathoth, and he received God's word during the reigns of Judah's last kings (Jeremiah 1:1-3). His ministry stretched through a painful season as Jerusalem moved towards destruction and exile.
So, when people ask who wrote Jeremiah, the simplest answer is Jeremiah. The book records the messages God gave him, his warnings to Judah, his prayers, his suffering, and his hope that God would one day restore his people. It is not simply a biography written about Jeremiah. Much of it carries the message entrusted to him (Jeremiah 1:7-9).
The words of Jeremiah the son of Hilkiah, of the priests that were in Anathoth in the land of Benjamin:
Baruch wrote down Jeremiah's words
Jeremiah did not always put the words on the page himself. During the reign of Jehoiakim, God told him to gather the messages he had received and have them written on a scroll. Jeremiah called Baruch, the son of Neriah, and dictated the words to him (Jeremiah 36:1-4).
Baruch was more than a casual helper. He read Jeremiah's scroll publicly in the temple when Jeremiah could not go himself, and he stayed connected to the prophet through dangerous days in Judah (Jeremiah 36:5-8; Jeremiah 43:3). In today's terms, Baruch acted as Jeremiah's scribe or secretary, faithfully recording the prophet's message.
Then Jeremiah called Baruch the son of Neriah: and Baruch wrote from the mouth of Jeremiah all the words of the Lord, which he had spoken unto him, upon a roll of a book.
The scroll was rewritten and expanded
King Jehoiakim reacted angrily when the scroll was read to him. He cut it up and burned it. But the destruction of that first copy did not end Jeremiah's message. God told Jeremiah to take another scroll, and Baruch wrote the words again as Jeremiah dictated them (Jeremiah 36:23; Jeremiah 36:27-28).
The new scroll contained the earlier words and further messages as well (Jeremiah 36:32). That helps explain why the book is substantial and varied. It gathers preaching, personal events, prayers, letters, and prophecies spoken across many years, rather than reading like one speech delivered on a single day.
Then took Jeremiah another roll, and gave it to Baruch the scribe, the son of Neriah; who wrote therein from the mouth of Jeremiah all the words of the book which Jehoiakim king of Judah had burned in the fire: and there were added besides unto them many like words.
What about the final shape of the book?
Some parts of Jeremiah tell events in the third person, and the closing chapter reports later history, including the release of King Jehoiachin in Babylon (Jeremiah 52:31-34). Scripture does not name the person who arranged every part into the final form now found in the book.
That leaves room for the reasonable possibility that Baruch, or another faithful editor, helped preserve and organise Jeremiah's material. Still, the book itself consistently points back to Jeremiah and the word God gave through him. Reading it with that in mind helps: listen for God's call to turn back to him, his grief over sin and suffering, and his promise of a new covenant (Jeremiah 31:31-33).