Mordecai (/ˈmɔːrdɪkaɪ, mɔːrdɪˈkeɪaɪ/; also Mordechai; Hebrew: מָרְדֳּכַי, Modern: Mŏrdoḵay, Tiberian: Mārdoḵay, IPA: [moʁdeˈχaj]) is one of the main personalities in the Book of Esther in the Hebrew Bible. He is the cousin and guardian of Esther, who became queen of Persia under the reign of Ahasuerus (Xerxes I). The king's grand vizier Haman is offended by Mordecai because of his refusal to bow before him, and when confronted, Mordecai's excuse was simply that he was a Jew. Consequently, Haman plots to have all of Persia's Jews killed, and eventually convinces Ahasuerus to permit him to do so. Mordecai's loyalty and bravery are highlighted throughout the story as he convinces Esther to use her position as queen to foil the plot of Haman, the king's vizier, to exterminate the Jewish people. His story is celebrated in the Jewish holiday of Purim, which commemorates his victory.

The historicity of Mordecai and the other characters in the Book of Esther, is a topic of longstanding debate. Traditionally seen as historical characters, there are scholars which still ascribe to this view. A popular theory frequently discussed in scholarship suggests that the Book of Esther serves as an etiology for Purim, with Mordecai and Esther representing the Babylonian gods Marduk and Ishtar in a historicized Babylonian myth or ritual. The identification of Mordecai with a Persian official named "Marduka" mentioned in an inscription from the reign of Xerxes I is debated, with some scholars rejecting the connection while others support it due to the commonality of name and office. Even if the Marduka who was mentioned in the inscription was not Mordecai himself it shows the name was in use during that period.

Mordecai resided in Susa (Shushan or Shoushan), the metropolis of Persia (now Iran). He adopted his orphaned cousin (Esther 2:7), Hadassah (Esther), whom he brought up as if she were his own daughter. When "young virgins" were sought, she was taken into the presence of King Ahasuerus and was made queen in the place of the exiled queen Vashti. Subsequently, Mordecai discovered a plot of the king's chamberlains Bigthan and Teresh to assassinate the king. Because of Mordecai's vigilance, the plot was foiled.

Haman the Agagite had been raised to the highest position at court. In spite of the king's decree that all should prostrate themselves before Haman, Mordecai refused to do so. Haman, stung by Mordecai's refusal, resolved to kill not only Mordecai but all Jewish exiles throughout the Persian empire, and won the king's permission to carry out his plan. Mordecai communicated Haman's scheme to Queen Esther, who used her favor with the king to reverse the scheme, leading the king to authorize Jews to kill their enemies, which they did.

During all this, the king had happened to learn of Mordecai's service in foiling the assassination plot and had asked Haman how a person who did a great service to the king should be honored. Haman answered, thinking the question was about him; and the king followed this advice, and honored Mordecai, and eventually made Mordecai his chief advisor. Haman was executed on gallows that he had set up for Mordecai. The feast of Purim celebrates these reversals of fortune.

Although the details of the setting are entirely plausible and the story may even have some basis in actual events, some think that the book of Esther is a novella rather than history. Persian kings did not marry outside of seven Persian noble families, making it unlikely that there was a Jewish queen Esther, and in any case the historical Xerxes's queen was Amestris.

There is a scholarly conjecture that the story was created to justify the Jewish appropriation of an originally non-Jewish feast. The festival which the book explains is purim, which is explained as meaning "lot", from the Babylonian word puru. There are wide-ranging theories regarding the origin of Purim: one popular theory says festival has its origins in a historicized Babylonian myth or ritual in which Mordecai and Esther represent the Babylonian gods Marduk and Ishtar, others trace the ritual to the Persian New Year, and scholars have surveyed other theories in their works. Some scholars have defended the story as real history, but others have said the attempt to find a historical kernel to the narrative "is likely to be futile".

The name "Mordecai" is of uncertain origin but is considered identical to the name Marduka or Marduku (Elamite: 𒈥𒁺𒋡), attested as the name of up to four Persian court officials in thirty texts (the Persepolis Administrative Archives) from the period of Xerxes I and his father Darius.