The Kitáb-i-Íqán (Persian: كتاب ايقان, Arabic: كتاب الإيقان, lit. 'Book of Certitude'; from the Arabic íqán, "to know with certitude"), also known as the Book of Íqán or simply The Íqán, is a book by Baháʼu'lláh, the founder of the Baháʼí Faith, and the principal doctrinal work of that religion. Composed in Baghdad over the course of two days and nights in January 1861, partly in Persian and partly in Arabic, the work was written in response to a series of questions posed by Ḥájí Mírzá Siyyid Muḥammad, a maternal uncle of the Báb, concerning the signs of the appearance of the promised one of Islam. Scholars in Islamic studies have treated the work as a major piece of Persian-language Quranic exegesis. Todd Lawson, writing in a Routledge volume on the Bahá'í tradition, characterizes it as "primarily a work of exegesis" and as one of Bahá'u'lláh's two most important books; the English Orientalist Edward Granville Browne praised the work as "a work of high merit, of vigorous style, lucid in argument, and convincing in its proofs;" and Christopher Buck, a scholar of religion specializing in Bahá'í and Quranic studies, has described it as "the most influential Quran commentary in Persian outside the Muslim world," on account of its international circulation.

The Kitáb-i-Íqán sets forth the foundational discourse of the Baháʼí Faith on the unity of religions, the progressive nature of revelation, and the spiritual reading of eschatological symbols. The first part argues that the religions are interrelated and that each major monotheistic religion, often in veiled language, foretells the advent of the next; the second offers theological and rational proofs of the mission of the Báb, including the passage known as the "Tablet of the True Seeker." The work expounds the meaning of such symbolic terms as Return, Resurrection, the Seal of the Prophets, and the Day of Judgment, and recasts each as descriptions of inner spiritual states rather than literal physical events.

Bahá'í tradition treats the work as second in importance only to the Kitáb-i-Aqdas and as a complement to the Báb's Persian Bayán. Shoghi Effendi, who translated the book into English in 1931, characterized it as setting forth "the Grand Redemptive Scheme of God" and as the work that, more than any other in the Bahá'í corpus, lays "a broad and unassailable foundation for the complete and permanent reconciliation" of the world's religions. Within scholarship of religion the book has been studied within the genre of Islamic Qurʼanic commentary (tafsír) and of Shiʻi eschatological hermeneutics, and is read by Bahá'ís devotionally and as a foundational text in study circles.

The work has attracted sustained scholarly attention as a contribution to Islamic Qurʼanic exegesis and Shiʻi eschatological hermeneutics, and is used devotionally in Bahá'í communities worldwide; it has been translated into over thirty languages.

The Kitáb-i-Íqán was composed by Baháʼu'lláh in 1861 while he was living as an exile in Baghdad, then a province of the Ottoman Empire. The book was a response to questions posed by Ḥájí Mírzá Siyyid Muḥammad, a maternal uncle of the Báb, who at that time had not accepted the messianic claim of his nephew. He had been perplexed to hear that the promised one of Islam was a member of his own family; on being told that this was the same objection voiced by the uncle of the prophet Muhammad, he resolved to investigate the matter. In 1861 he travelled to Karbala, Iraq, to visit his brother, Ḥájí Mírzá Ḥasan-ʻAlí, and from there to Baghdad to meet Baháʼu'lláh. There he posed four written questions concerning the signs of the appearance of the promised one, the meaning of resurrection and judgement, and the seal of prophethood. Approximately 200 pages in length in the original languages, the work was composed over no more than two days and two nights, around January 15, 1861. A letter the questioner wrote to his son from Baghdad on January 17, 1861 provides the documentary basis for this dating. The book was originally known as the Risálih-i-Khálúyih ("Treatise for the Maternal Uncle") and the Risálih-i-Istidláliyyih ("Treatise of Proofs"); Baháʼu'lláh later, while residing in ʻAkká, gave the work the title Íqán, "to know with certitude."

When Baháʼu'lláh composed the Kitáb-i-Íqán he had not yet publicly declared his own mission, which he would announce at the Garden of Ridván in 1863. He had received an inaugural revelatory experience some ten years earlier, in 1852, while imprisoned in the Síyáh-Chál (lit. "black pit"), a dungeon in Tehran; references to his own station as a Manifestation of God therefore appear in the Íqán only in veiled form. Christopher Buck has likened this feature of the book to the so-called "messianic secret" of the Gospel of Mark in the Christian New Testament, and has argued that it is one of the central interpretive keys to the work.

In the years immediately following its composition, the Kitáb-i-Íqán circulated widely in manuscript among the Bábí community of Iran. Its prose was fluent and accessible to ordinary readers without formal religious education, and, according to Smith, attracted many readers within the Bábí community toward Baháʼu'lláh. The work is regarded as having played a significant role in revitalizing a community devastated by the execution of the Báb in 1850 and the subsequent state persecution. Together with other works of Baháʼu'lláh's decade-long Iraqi exile – including the Hidden Words, the Seven Valleys, the Four Valleys, and the Gems of Divine Mysteries – it stands among the major writings of his Baghdad period and addresses the doctrinal foundations of the new religion in a register distinct from the mystical, ethical, and metaphysical treatises of the same period. Within Bábí circles, the book served both to clarify and vindicate the claims of the Báb and to foster the renewal of the community..

The Kitáb-i-Íqán was probably the first work of Baháʼu'lláh to appear in print. A lithographed edition was produced by relatives of the Báb (the Afnán family) in Bombay, India, around 1882 by the Ḥasaní Zívar Press; a second lithograph followed in 1893 in the calligraphy of Mishkín-Qalam, a noted Bahá'í calligrapher. Prior to lithographic publication the book had circulated for some two decades in handwritten copies, and Baháʼu'lláh wrote at least one tablet to a follower addressing scribal errors in those manuscripts and overseeing the production of authoritative master copies. The first formal typeset edition was published in Egypt in 1900, and subsequent editions derived from this version. In the first Bombay lithograph, a number of Qurʼanic verses had been quoted inexactly; at Baháʼu'lláh's instruction these were brought into conformity with the Qurʼanic original in later editions.