Mahakali (Sanskrit: महाकाली, romanized: Mahākālī) is a deity common to Hinduism and Buddhism. In Hinduism, she is the goddess of time and death in the goddess-centric tradition of Shaktism. She is also known as the supreme being in various Hindu tantras and Puranas.

Similar to Kali, Mahakali is a fierce goddess associated with universal power, time, life, death, and both rebirth and liberation. She is the consort of Mahakala, the god of consciousness, the basis of reality and existence. Mahakali, in Sanskrit, is etymologically the feminised variant of Mahakala, or Great Time (which is also interpreted as Death), Shiva in Hinduism.

Mahakali's origin is found in various Puranic and Tantric Hindu scriptures (Shastras). In the texts of Shaktism, she is variously portrayed as the Adi-Shakti, the Primeval Force of the Universe, identical with the Ultimate Reality, or Brahman. She is also known as the (female) Prakriti or the world as opposed to the (male) Purusha or the consciousness, or as one of three manifestations of Mahadevi (The Great Goddess) that represent the three Gunas or attributes in Samkhya philosophy. In this interpretation, Mahakali represents Tamas or the force of inertia. Devi Mahatmya ("Greatness of the Goddess"), a later interpolation into the Markandeya Purana, is considered to be one of the core texts of Shaktism (the branch of Hinduism which considers Durga to be the highest aspect of Godhead). It assigns a different form of the Goddess (Mahasaraswati, Mahalakshmi, and Mahakali) to each of the three episodes therein. Here, Mahakali is assigned to the first episode. She is described as an abstract energy, the yoganidra of Vishnu.

Mahakali is most often depicted in blue/black complexion in popular Indian art.

Her most common four-armed iconographic image shows each hand carrying variously a crescent-shaped khadha, a sword, a trishula (trident), a severed head of a demon and a bowl or skull-cup (kapala) catching the blood of the severed head. Her eyes are described as red with intoxication and in absolute rage, her hair is shown disheveled, small fangs sometimes protrude out of her mouth and her tongue is lolling. The blood of the demons she slays drips out of her lolling tongue, having consumed it. She is adorned with a garland consisting of the heads of demons she has slaughtered, variously enumerated at 108 (an auspicious number in Hinduism and the number of countable beads on a japa mala, similar to a rosary, for repetition of mantras) or 50, which represents the letters of the Sanskrit alphabet, Devanagari, and wears a skirt made of demon arms.

Her ten-headed (dashamukhi) image is known as the 10 Mahavidyas Mahakali, and in this form she is said to represent the ten Mahavidyas or "Great Wisdom (goddess)s". She is sometimes shown sitting on a flaming grave or a rotting corpse. Her complexion is described as that of the night sky, devoid of stars. She is depicted in this form as having ten heads, thirty flaming eyes, ten arms, and ten legs but otherwise usually conforms to the four armed icon in other respects. Each of her ten hands is carrying an implement which varies in different accounts, but each of these represent the power of one of the devas, and are often the identifying weapon or ritual item of a given deva. The implication is that Mahakali subsumes and is responsible for the powers that these deities possess and this is in line with the interpretation that Mahakali is identical with Brahman. While not displaying ten heads, an ekamukhi (one-headed) image may be displayed with ten arms, signifying the same concept: the powers of the various gods coming only through her grace.

In either one of these images she is shown standing on the prone, inert body of Shiva. This is interpreted in various ways but the most common is that Mahakali represents Shakti, the power of pure creation in the universe, and Shiva represents pure Consciousness which is inert in and of itself. While this is an advanced concept in monistic Shaktism, it also agrees with the Nondual Trika philosophy of Kashmir, popularly known as Kashmir Shaivism and associated most famously with Abhinavagupta. There is a colloquial saying that "Shiva without Shakti is Shava" which means that without the power of action (Shakti) that is Mahakali (represented as the short "i" in Devanagari) Shiva (or consciousness itself) is inactive; Shava means corpse in Sanskrit and the play on words is that all Sanskrit consonants are assumed to be followed by a short letter "a" unless otherwise noted. The short letter "i" represents the female power or Shakti that activates Creation. This is often the explanation for why she is standing on Shiva, who is her husband in Shaktism, and also the Supreme Godhead in Shaivism. Another understanding is that the wild destructive Mahakali can only stop her fury in the presence of Shiva the God of Consciousness, so that the balance of life is not completely overrun over by wild nature.

According to Jainism, Mahākālī is a sasana-devi, also called Yakshini, which is a class of female guardian deities. Some traditions state that she is the Yakṣiṇī of the 5th Tirthankara Sumatinatha, while others say she is the attendant deity of Suvidhinatha, the 9th Tirthankara. Mahākālī also refers to one of the sixteen Jain goddesses of learning, known as Vidya Devis.