ʻAbdu'l-Bahá KBE (/əbˈdʊl bəˈhɑː/; Persian: عبد البهاء, IPA: [ʔæbdolbæhɒːʔ];, 23 May 1844 – 28 November 1921), born ʻAbbás (Persian: عباس, IPA: [ʔæbːɒːs]), was the eldest son of Baháʼu'lláh, founder of the Bahá’í Faith, who designated him to be his successor and head of the Baháʼí Faith from 1892 until 1921. ʻAbdu'l-Bahá was later cited as the last of three "central figures" of the religion, along with Baháʼu'lláh and the Báb, and his writings and authenticated talks are regarded as sources of Baháʼí sacred literature.

He was born in Tehran to an aristocratic family. At the age of eight, his father was imprisoned during a government crackdown on the Bábí Faith and the family's possessions were looted, leaving them in virtual poverty. His father was exiled from their native Iran, and the family established their residence in Baghdad in Iraq, where they stayed for ten years. They were later called by the Ottoman state to Istanbul before entering another period of confinement in Edirne and finally the prison-city of ʻAkká (Acre). ʻAbdu'l-Bahá remained a prisoner there until the Young Turk Revolution freed him in 1908 at the age of 64. He then made several journeys to the West to spread the Baháʼí message beyond its middle-eastern roots, but the onset of World War I left him largely confined to Haifa from 1914 to 1918. Following the war, the openly hostile Ottoman authorities were replaced by the British Mandate over Palestine, during which time he was appointed a Knight Commander of the Order of the British Empire for his help in averting famine following the war.

In 1892, ʻAbdu'l-Bahá was appointed in his father's will to be his successor and head of the Baháʼí Faith. His Tablets of the Divine Plan galvanized Baháʼís in North America to spread the Baháʼí teachings to new territories, and his Will and Testament laid the foundation for the current Baháʼí administrative order. Many of his writings, prayers and letters are extant, and his discourses with the Western Baháʼís emphasize the growth of the religion by the late 1890s.

ʻAbdu'l-Bahá's given name was ʻAbbás. Depending on context, he would have gone by either Mírzá ʻAbbás (Persian) or ʻAbbás Effendi (Turkish), both of which are equivalent to the English Sir ʻAbbás. During most of his time as head of the Bahá'í Faith, he used and preferred the title of ʻAbdu'l-Bahá ("servant of Bahá", a reference to his father). He is commonly referred to in Baháʼí texts as "The Master".

ʻAbdu'l-Bahá was born in Tehran, Persia (now Iran) on 23 May 1844 (5th of Jamadiyu'l-Avval, 1260 AH), the eldest son of Baháʼu'lláh and Navváb. He was born on the same night on which the Báb declared his mission. Given the name ʻAbbás at birth, he was named after his grandfather Mírzá ʻAbbás Núrí, a prominent and powerful nobleman. ʻAbdu'l-Bahá's early years were shaped by his father's prominent role within the Bábí community. As a child, he fondly recalled interactions with the Bábí, Táhirih, describing how she would take him on her knee, caress him, and engage in heartfelt conversations, leaving a lasting impression on him. His childhood was characterized by happiness and carefree moments. The family's residences in Tehran and the countryside were not only comfortable but also beautifully adorned. Alongside his younger siblings – a sister, Bahíyyih, and a brother, Mihdí – he experienced a life of privilege, joy, and comfort. ʻAbdu'l-Bahá loved playing in the gardens with his younger sister, fostering a strong bond between them. During his formative years, ʻAbdu'l-Bahá observed his parents' commitment to various charitable endeavors, including the conversion of part of their home into a hospital ward for women and children.

Due to a life largely marked by exile and imprisonment, ʻAbdu'l-Bahá had limited opportunities for formal schooling. In his youth, it was customary for children of nobility, including ʻAbdu'l-Bahá, not to attend conventional schools. Instead, noblemen typically received a brief education at home, focusing on subjects such as scripture, rhetoric, calligraphy, and basic mathematics, with an emphasis on preparing for life within royal courts.

ʻAbdu'l-Bahá spent only a short period at a traditional preparatory school at the age of seven for a single year. His mother and uncle took on the responsibility of his early education, but the primary source of his learning was his father. In 1890 Edward Granville Browne described ʻAbdu'l-Bahá, saying that "one more eloquent of speech, more ready of argument, more apt of illustration, more intimately acquainted with the sacred books of the Jews, the Christians, and the Muhammadans...could scarcely be found..."

According to contemporary accounts ʻAbdu'l-Bahá was an eloquent and charming child. At the age of seven, he faced a severe health challenge when he contracted tuberculosis, and his prognosis suggested death. Though the illness abated, this marked the beginning of a lifelong struggle with recurrent bouts of various illnesses that would persist throughout his life.