Mujaddid () in Islam is a reformer who is given the task of removing errors that have occurred among Muslims. Their job is to show people the great religious truths which the Muslim community will be asked to face. According to the popular Muslim tradition, it refers to a person who appears at the turn of every century of the Islamic calendar to revive Islam, cleansing it of  extraneous elements and restoring it to its pristine purity. In contemporary times, a mujaddid is looked upon as the greatest Muslim of a century.

Ikhtilaf (disagreements) exist among vatious hadith specialists. Scholars and historians like Al-Dhahabi and Ibn Hajar al-Asqalani have interpreted that the term mujaddid can also be understood as plural, thus referring to a group of people.

The Arabic word mujaddid means "reformer", "renovator", "reviver", "renewer" or "regenerator". It is someone who revives and renovates the religion. The concept of tajdid (renewal or revival) and the term mujaddid come rather from a hadith, a statement of the Prophet Muhammad. This hadith was written down by Abu Dawood in his Sunan, one of the six authoritative Sunni collections of the Prophet's statements. In this hadith, the Prophet says:

This means reform is in the essential nature of Islam and Muslims are called all the time to work hard to make new ideas cope with tradition. It also means that not everything in the Muslim tradition is useful and good for this modern age; there are certain things that were possible in the past but are no longer relevant today. Slavery would be a prime example.

The concept of tajdid in Islamic thought 
Tajdid (renewal) in Islamic thought means renewing the ideology representing the intellectual product of Muslims in the fields of science, knowledge and ijtihad to interpret Islam and understand and explicate its rulings.

Al-Suyuti mentioned in his book Al-Jami' al-Sagheer, "Renewing religion means renewing its guidance, clarifying its truth and precedence, refuting the innovations and extremism presented to its followers or their reluctance in upholding it, and following its rules in managing the interests of the people and the law of society and civilization."

Among the most manifest aspects of tajdid (renewal) in Islamic thought is the renewal of Islamic sciences as follows:
 The science of Islamic doctrine.
 The Principles of Islamic jurisprudence.
 The science of Jurisprudence.
 The science of the sunnah.
 The science of Qur'anic exegesis (tafsir).
 The science of Purification and Code of Conduct (Sufism).
 The biography of the Prophet and Islamic history.

Mujaddids can include prominent scholars, pious rulers and military commanders.

The reformers in Islam
There is no formal mechanism for designating a mujaddid. The persons of this list are claimed to be Mujaddid.

While there is no formal mechanism for designating a mujaddid in Sunni Islam, there is often a popular consensus.

1th Century AH
 Al-Hasan al-Basri (21–110 A.H./642–728 A.D.)
 Umar ibn Abd al-Aziz (63–101 A.H./682–720 A.D.)
 Abu Hanifah (80–150 A.H./702–772 A.D.)
 Malik ibn Anas (93–179 A.H./711–795 A.D.)

2th Century AH
 Muhammad ibn Idris al-Shafi'i (150–204 A.H./767–820 A.D.)
 Ahmad ibn Hanbal (164–241 A.H./781–855 A.D.)

3th Century AH
 Abu al-Hasan al-Ash'ari (260–324 A.H./873–935 A.D.)
 Abu Mansur al-Maturidi (238–333 A.H./852–944 A.D.)
 Abu Ja'far al-Tahawi (239–321 A.H./853–933 A.D.)
 Ibn Surayj (249–306 A.H./864–918 A.D.)

4th Century AH
 Abu Bakr al-Baqillani (338–403 A.H./950–1013 A.D.)
 Ibn Furak (330–406 A.H./941–1015 A.D.)
 Al-Hakim al-Nishapuri (321–405 A.H./933–1014 A.D.)
 Abu Hamid al-Isfarayini (344–406 A.H./955–1015 A.D.)
 Abu al-Tayyib Sahl al-Sa'luki (000–404 A.H./000–1013/14 A.D.)
 Ibn Hazm al-Andalusi (384–456 A.H./994–1064 A.D.)
 Abu al-Ma'ali al-Juwayni (419–478 A.H./1028–1085 A.D.)

5th Century AH
 Abu Hamid al-Ghazali (450–505 A.H./1058–1111 A.D.)
 Ibn al-Jawzi (509/510–597 A.H./1116–1201 A.D.)
 Ahmad al-Rifa'i (512–578 A.H./1118–1182 A.D.)

6th Century AH
 Fakhr al-Din al-Razi (543/44–606 A.H./1149–1209 A.D.)
 Abu al-Qasim al-Rafi'i (555–623 A.H./1160–1226 A.D.)
 Al-Baydawi (000–685 A.H./000–1286 A.D.)
 Al-'Izz ibn 'Abd al-Salam (577–660 A.H./1181/82–1262 A.D.)

7th Century AH
 Ibn Daqiq al-'ld (625–702 A.H./1228–1302 A.D.)
 Ibn 'Ata' Allah al-Sakandari (658–709 A.H./1259–1309 A.D.)
 Ibn Battuta (703–779 A.H./1304–1377 A.D.)
 Abu Ishaq al-Shatibi (720–790 A.H./1320–1388 A.D.)
 Al-Taftazani (722–793 A.H./1322–1390 A.D.)

8th Century AH
 Siraj al-Din al-Bulqini (724-805 A.H./1324-1403 A.D.)
 Zain al-Din al-'Iraqi (725-806 A.H./1325-1404 A.D.)
 Ibn Khaldun (732-808 A.H./1332-1406 A.D.)

9th Century AH
 Jalal al-Din al-Suyuti (849–911 A.H./1445–1505 A.D.)
 Zakariyya al-Ansari (823–926 A.H./1420–1520 A.D.)

10th Century AH
 Shams al-Din al-Ramli (919–1004 A.H./1513–1596 A.D.)
 Khayr al-Din al-Ramli (993–1081 A.H./1585–1671 A.D.)
 Ahmad Sirhindi (971–1034 A.H./1564–1624 A.D.)

11th Century AH
 Aurangzeb (1068–1118 A.H./1658–1707 A.D.)
 'Abdallah ibn 'Alawi al-Haddad (1044–1132 A.H./1634–1719 A.D.)
 Shah Waliullah Dehlawi (1114–1176 A.H./1703–1762 A.D.)

12th Century AH
 Murtada al-Zabidi (1145–1205 A.H./1732–1791 A.D.)
 Ahmad ibn 'Ajiba (1160–1224 A.H./1747–1809 A.D.)
 Shah Abdul Aziz Dehlawi (1159–1239 A.H./1746–1824 A.D.)

Thirteenth Century (November 14, 1882)
Muhammad Abduh (1849–1905)
Said Nursî (1878–1960)

Fourteenth Century (November 21, 1979)
Ahmed Raza Khan ( 1865 - 1921)
Muhammad Nasiruddin al-Albani (1914-1999)

Controversial figures 
 Ibn Taymiyyah (661–728 A.H./1263–1328 A.D.)
 Usman dan Fodio (1167–1233 A.H./1754–1817 A.D.)

 Al-Albani (1332–1420 A.H./1914–1999 A.D.)

Notes

References 
 Tuhfat al-Muhtadin bi Akhbar al-Mujaddidin () by Jalal al-Din al-Suyuti.
 Mausu'at A'lam al-Mujaddidin fi al-Islam () by Samih Kurayyim.
 Mujaddid - Islamic Encyclopedia

Other websites 
 Concept of a Mujaddid and the past and present ones

Islam
Arabic words and phrases
Religious leaders