Why doesn't Surah at-Tawbah start with Bismillah?

Surah at-Tawbah doesn't start with Bismillah because it was never revealed or written with one, and the Companions kept it exactly as they received it. Scholars give two main explanations: it opens with a stern declaration that doesn't fit the words of mercy, and it may be a continuation of the surah before it, al-Anfal.

The only surah without it

Every one of the 114 surahs begins with "Bismillahir-Rahmanir-Rahim" (In the name of Allah, the Most Gracious, the Most Merciful), except chapter 9, at-Tawbah ("Repentance"), also called Bara'ah ("Disassociation"). The count still works out, because Bismillah appears twice in Surah an-Naml: at the start and inside verse 27:30, in Solomon's letter to the Queen of Sheba.

The most important point is simple: the Companions copied the Quran exactly as the Prophet ﷺ taught it, and he never had Bismillah written at the head of this surah. Nobody added or removed anything on their own. See Was the Quran changed over time?

Explanation 1: a declaration, not a greeting

The surah opens with a formal ultimatum to tribes who had repeatedly broken their peace treaties with the Muslims. Ali ibn Abi Talib is reported to have said that Bismillah is a promise of safety, while this surah came down as a withdrawal of that safety from treaty-breakers. A strong warning doesn't open with a greeting of mercy.

Explanation 2: it continues al-Anfal

Uthman ibn Affan is reported to have explained that the themes of al-Anfal (chapter 8) and at-Tawbah are so similar, both about treaties, battles and loyalty, that the Prophet ﷺ died without clearly saying whether they were one surah or two. So the Companions placed them side by side, without Bismillah between them, but as separate chapters. This also connects to why the surahs follow a set order rather than the order of revelation. See Why isn't the Quran in chronological order?

Mercy is still all through it

Despite its stern opening, the surah is named "Repentance," and it keeps the door of tawbah open even for its enemies. It ends with one of the most loving descriptions of the Prophet ﷺ:

لَقَدْ جَآءَكُمْ رَسُولٌ مِّنْ أَنفُسِكُمْ عَزِيزٌ عَلَيْهِ مَا عَنِتُّمْ حَرِيصٌ عَلَيْكُم بِٱلْمُؤْمِنِينَ رَءُوفٌ رَّحِيمٌ "There has certainly come to you a Messenger from among yourselves. Grievous to him is what you suffer; he is concerned over you and to the believers is kind and merciful." Quran 9:128

What do I say when I start reciting it?

Most scholars say you don't recite Bismillah at the start of at-Tawbah, whether you're continuing from al-Anfal or starting there. Just say "A'udhu billahi minash-shaytanir-rajim" and begin. Some reciters allow Bismillah if you start in the middle of the surah. See Do I have to say "A'udhu billah" before reading the Quran?

Key points

At-Tawbah is the only surah with no opening Bismillah.
It was never written with one, and nobody changed that.
Reasons given: its stern opening, and its link to al-Anfal.
Start it with "A'udhu billah," not Bismillah.

Tip: Notice the missing Bismillah when you reach page 187 of a standard mushaf. Moments like this are a nice reminder that the Quran in your hands is laid out exactly as it was 1,400 years ago.