Imam Aḥmad bin Muḥammad bin Ḥanbal Abū ʿAbd Allāh al-Shaybānī (780–855 CE / 164–241 AH) (Arabic: احمد بن محمد بن حنبل ابو عبد الله الشيباني‎‎) was an important Muslim scholar and theologian. He is considered the founder of the Hanbali school of Islamic jurisprudence. Ibn Hanbal is one of the most celebrated Sunni theologians, often referred to as "Sheikh al-Islam," honour title given to the most esteemed doctrinal authorities in the Sunni tradition.

Imam Ibn Hanbal personified the theological views of the early orthodox scholars, including the founders of the other extant schools of Sunni fiqh.

Imam Ahmad ibn Hanbal was born in the city of Baghdad during the month of Rabi-ul Awwal 164 A.H. His father passed away either before he was born or shortly afterwards, and it was his mother who diligently brought up him up (single mother).

Imam Ahmad Ibn Hanbal studied extensively in Baghdad, and later traveled to further his education. He started learning jurisprudence (Fiqh) under the celebrated Hanafi judge, Abu Yusuf, the renowned student and companion of Imam Abu Hanifah. After finishing his studies with Abu Yusuf, ibn Hanbal began traveling through Iraq, Syria, and Arabia to collect hadiths, or traditions of the Prophet Muhammad. Ibn al-Jawzi states that Imam Ahmad had 414 Hadith masters whom he narrated from. With this knowledge, he became a leading authority on the hadith, leaving an immense encyclopedia of hadith, the al-Musnad. After several years of travel, he returned to Baghdad to study Islamic law under Al-Shafi'i. He became a mufti in his old age, but is remembered most famously, as the founder of the Hanbali madhab or school of Islamic law, which is now most dominant in Saudi Arabia, Qatar as well as the United Arab Emirates. Unlike the other three schools of Islamic jurisprudence (Hanafi, Maliki, and Shafi), the Hanbali madhab remained largely traditionalist or Athari in theology.

In addition to his scholastic enterprises, ibn Hanbal was a soldier on the Islamic frontiers (Ribat) and made Hajj five times in his life, twice on foot

Ahmad Ibn Hanbal died on Friday, 12 Rabi-ul-I, 241 AH/ 2nd August, 855 at the age of 74-75 in Baghdad, Iraq. Historians relate that his funeral was attended by 800,000 men and 60,000 women and that 20,000 Christians and of Jewish faith converted to Islam on that day.

Imam Hanbal was famously called before the Inquisition or Mihna of the Abassid Caliph al-Ma'mun. Al-Ma'mun wanted to assert the religious authority of the Caliph by pressuring scholars to adopt the Mu'tazila view that the Qur'an was created rather than uncreated. According to Sunni tradition, ibn Hanbal was among the scholars to resist the Caliph's interference and the Mu'tazila doctrine of a created Qur'an—although some Orientalist sources raise a question on whether or not he remained steadfast Ibn Hanbal's stand against the inquisition by the Mu'tazila (who had been the ruling authority at the time) led to the Hanbali school establishing itself firmly as not only a school of fiqh (legal jurisprudence), but of theology as well.

Due to his refusal to accept Mu'tazilite authority, ibn Hanbal was imprisoned in Baghdad throughout the reign of al-Ma'mun. In an incident during the rule of al-Ma'mun's successor, al-Mu'tasim, ibn Hanbal was flogged to unconsciousness. However, this caused upheaval in Baghdad and al-Ma'mun was forced to release ibn Hanbal. After al-Mu’tasim’s death, al-Wathiq became caliph and continued his predecessor's policies of Mu'tazilite enforcement and in this pursuit, he banished ibn Hanbal from Baghdad. It was only after al-Wathiqu's death and the ascent of his brother al-Mutawakkil, who was much friendlier to the more traditional Sunni beliefs, that ibn Hanbal was welcomed back to Baghdad.

Ibn Hanbal has been extensively praised for both his work in the field of prophetic tradition. Abdul-Qadir Gilani stated that a Muslim could not truly be a wali of Allah except that they were upon Ibn Hanbal's creed; despite praise from his contemporaries as well, Yahya ibn Ma'in noted that Ibn Hanbal never boasted about his achievements.