Zayn ad-Din (Arabic: زين الدين, also transliterated as Zinedine, Zeyneddin, Zain-ud-Din, Zainuddin, Zainaddin, etc.) is an Arabic name meaning "grace of the religion" and may refer to:

• Zayn al-Din Ali Kutchek (died 1168): son of Begtakin, Atabeg of Erbil and father of Muzaffar al-din Gokbori
• Zayn al-Din al-Amidi (died 1312), blind Iraqi scholar who invented a method for the blind to read
• Zayn al-Din Azraqi: Abul-Mahāsin Abu Bakr Zaynuddin Azraqi, or just Azraqi (died 1072), Persian poet
• Al-Ashraf Zein al-Din Abu al-Ma'ali ibn Shaban, or just Al-Ashraf Sha'ban (died 1377), Mamluk Sultan of Egypt
• Zainuddin, 15th-century Bengali poet
• Zainuddin bin Maidin (born 1939), Malaysian politician
• Zinedine Ferhat (born 1993), Algerian footballer
• Zayn al-Din Gorgani (1040–1136), Persian court physician
• Al-Salih Salah Zein al-Din Hajji II, or just Al-Salih Hajji (died c. 1389), Mamluk Sultan of Egypt
• Zayn al-Din al-Juba'i al'Amili (1506–1558), Shi'ite cleric and martyr
• Zayn al-Din Kitbugha: Al-Malik al-Adil Zayn-ad-Din Kitbugha Ben Abd-Allah al-Mansuri al-Turki al-Mughli, or just Al-Adil Kitbugha (died 1297), Mamluk Sultan of Egypt
• Zinédine Machach (born 1996), French footballer
• Zainuddin Makhdoom II (died 1583), Indian Islamic lawyer and author
• Sheikh Zainuddin, Bengali artist who rose to prominence during the British Raj
• Zineddine Mekkaoui (born 1987), Algerian footballer
• Zain al-Din Muhammad Abdul Hady, or just Zain Abdul Hady (born 1956), Egyptian information scientist and novelist
• Zainudin Nordin (born 1963), Singaporean politician
• Zine Eddine Sekhri, Algerian Paralympian athlete
• Zainuddin Shirazi (c. 1302 – c. 1370), Indian Sufi saint
• Zinedine Soualem (born 1957), French film actor
• Zaynitdin Tadjiyev (born 1977), Uzbek footballer
• Zayn al-Din Qaraja (c. 1279 – 1353), founder of the Dulkadirids
• Zinedine Zidane (born 1972), French footballer and manager
• Mohammad Ebrahim Zainuddin 'Ebbu' Ghazali, or just Mohammed Ghazali (1924–2003), Pakistani cricketer
• Qari Zainuddin Mehsud, or Qari Zain, Pakistani Taliban leader

• A. C. Zainuddin (1952–1999), Indian actor in Malayalam film
• Bakhtiyar Zaynutdinov (born 1998), Kazakhstani footballer
• Daim Zainuddin (born 1938), Malaysian economist and politician
• Hamzah Zainuddin (born 1957), Malaysian politician
• Mohamed Alí Seineldín (1933–2009), Argentine army colonel
• Nazira Zain al-Din (born 1908), Druze Lebanese scholar and women's rights activist
• Raya Zeineddine (born 1988), Syrian sports shooter

• Zeynəddin, village and municipality in the Babek Rayon of Nakhchivan, Azerbaijan
• Aşağı Zeynəddin, village and municipality in the Agdash Rayon of Azerbaijan
• Yuxarı Zeynəddin, village and municipality in the Agdash Rayon of Azerbaijan
• Zeyn ol Din, Fars, Iran
• Zeyn ol Din, Kermanshah, Iran
• Zeyn ol Din, Razavi Khorasan, Iran
• Zeinodin Caravanserai, located at Zein-o-din, Yazd, Iran

• Zeyn ol Dini, Lamerd, Fars Province, Iran