Tefillin (; Israeli Hebrew: תְּפִלִּין‎ / תְּפִילִּין‎; Ashkenazic pronunciation: [tfiˈlin]), or phylacteries, are a set of small black leather boxes with leather straps containing scrolls of parchment inscribed with verses from the Torah. Tefillin are worn by adult Jews during weekday morning prayers. In Orthodox and traditional communities, they are worn solely by men, while some Reform and Conservative (Masorti) communities allow them to be worn by both men and women. By traditional Jewish Law (halacha), women are exempt from most time-dependent positive commandments.

Although "tefillin" is technically the plural form (the singular being "tefillah"), it is often used as a singular as well. The arm-tefillah (or shel yad [literally "of the hand"]) is placed on the upper (non-dominant) arm, and the strap wrapped around the forelimb, hand and middle finger; while the head-tefillah (or shel rosh [literally "of the head"]) is placed between the eyes at the boundary of the forehead and hair. They are intended to fulfill the Torah's instructions to maintain a continuous "sign" and "remembrance" of the Exodus from Egypt, as they were originally worn all day, from sunrise to sunset.

The biblical verses often cited as referring to tefillin are obscure. Deuteronomy 11:18, for instance, does not designate explicitly what specifically to "bind upon your arm", and the definition of "totafot between your eyes" is not obvious. These details are delineated in the Oral Torah. At least as early as the 1st century CE, many Jews understood the verses literally and wore physical tefillin, as shown by archaeological finds at Qumran and a reference in the New Testament. However, Karaite Judaism understands the verses to be metaphorical.