What should I do if I accidentally eat something haram without realizing it?

If you genuinely didn't know and had no reasonable way of knowing that what you ate was haram, you are not sinful for it — Islamic law does not hold a person accountable for a mistake made without knowledge or intent, and there is no formal expiation (kaffarah) required specifically for accidentally eating haram food. The moment you realize what happened, the appropriate response is simply to stop, avoid it going forward now that you know, and make sincere istighfar (seeking Allah's forgiveness) as a matter of general good practice, not because you're formally guilty of a sin.

This differs meaningfully from knowingly and deliberately eating something haram, which does carry sin and calls for genuine repentance (tawbah). Scholars also note that if you're mid-meal and only then discover a specific dish or ingredient is haram, you should simply stop eating it at that point — you're not obligated to make yourself sick or take any extreme action, just to discontinue once aware.

Source: general fiqh principle that accountability requires knowledge and intent (see the broader hadith principle "my Ummah is forgiven for mistakes, forgetfulness, and what they are compelled to do," cited across fiqh literature though its precise chain is discussed among hadith scholars); see SeekersGuidance and Islamweb fatwas on eating haram food unintentionally.