In Roman mythology, Mefitis (or Mephitis; Mefite in Italian) was an Italic goddess. In Roman literature, she is often associated with the poisonous gases emitted from the ground in swamps and volcanic vapors. Consequently, she is sometimes cast as the goddess of intoxication, though modern scholarship generally associates her with medicinal, agricultural, or mediatory roles instead.

Mefitis was a Samnite and Oscan goddess worshipped from the 7th-century BCE to the 2nd-century CE whose cult was primarily concentrated in the southwest of Central Italy. In ancient literature, she is often associated with the foul-smelling gases of the earth. For instance, the 5th-century author Servius the Grammarian claims that any "foul scent is thought to properly belong to Mefitis," who he claims was "born from sulfur water." Despite the plethora of ancient literary accounts documenting her supposed association with noxious fumes and geologic activity, the archaeological evidence from her sanctuaries does not necessarily support her supposed status as goddess of intoxication. The archaeologist Adriano La Regina notes that sulfurous water, though present in her sanctuary at Ampsanctus, is absent from the sanctuaries at Saepinum and Rossano. Battiloro suggests that Mefitis was likely conceptualized as a more benevolent deity, arguing that her epithet Aravinna—which may connect to Latin arva ("soil")—implies a link with crops, and therefore may indicate that Mefitis served to protect agriculture in some capacity.

According to the archaeologist Ingrid Edlund-Berry, it is likely that the divine role of Mefitis involved healing, as sulfur is ascribed medicinal and purificatory prosperities in Roman writings. For instance, Pliny the Elder writes that sulfur "has a place in religious ceremonies" ("habet et in religionibus locum") and is utilized for "purifying houses by fumigation" ("ad expiandas suffitu domos"). Moreover, the archaeologist Barbro Santillo Frizell notes that Virgil describes the usage of water to treat sheep scab, stating that "foul scab attacks sheep" and thus "the keepers bathe the whole flock in fresh streams." Geographically, the Ansanto sanctuary is situated nearby transhumance routes utilized by ancient farmers, which may indicate that the site functioned to treat livestock. It is theorized that Mefitis was originally a goddess of underground sources, such as natural springs—the fact that many of these springs were sulfurous led to her association with noxious gases. However, the classicist Allison Griffith argues that there is no archaeological or literary corroborating the usage of these sites for healing or bathing. Regarding Rossano specifically, Griffith notes that the known votive assemblage at the site consists of militaristic objects such as greaves or spear points and also bronze figurines, none of which have any associations with healing. The archaeologist Elena Isayev argues that Mefitis was likely worshipped in various capacities depending upon the context of the needs of her suppliants.

Placidus claims that she was worshipped in "many places" throughout Italy and that she had a sacred lake in Lucania from which a sulphuric odor was emitted. Tacitus mentions another supposed sanctuary to Mefitis in Cremona, which he claims survived the destruction of the city after the Second Battle of Bedriacum as it was safeguarded by "either its position or its deity." The classicist Saskia Roselaar proposes that Mefitis—a goddess primarily worshipped in southern Italy—may have acquired suppliants in the northern Italian town of Cremona due to the arrival of migrants colonists. Mirabella Eclano (Irpinia) was the site of another sanctuary. An inscription on the wall of the House of the Great Fountain in Pompeii mentions a festival celebrating Mefitis, organized by the gens Mamia.

Her main shrine was located at the lake Ampsanctus in Samnium. Today, it lies near the village of Rocca San Felice in the province of Avellino, itself in the Campania region. Cicero and Pliny the Elder both describe the lake of Ampsanctus as deadly, with Pliny more specifically stating that her sanctuary was afflicted with toxic natural gases and that anyone who entered her temple would soon perish. Servius designates the areas the "navel of Italy" ("umbilicum Italiae") and claims that the ancient Romans would pay homage to the goddess by performing animal sacrifices using the fissure's deadly gases. Many clay votive statuettes and other objects found in the Ansanto valley depict wild boars, perhaps indicating that these animals were particularly sacred to the goddess. Virgil connects the sanctuary to the underworld in the Aeneid, describing the site as a "breathing place of savage Dis" and a "vast gorge from which Acheron bursts forth." Virgil additionally claims that an Erinys hides by the lake, where they relieve "earth and heaven." The site of Ansanto is now known to output significant quantities of carbon dioxide via degassing processes, which has prevented the growth of most plant species within the surrounding area.

Varro, a 1st-century BCE Roman polymath, mentions a grove of Mefitis ("lucus Mefitis") by the Esquiline hill in Rome—a claim which is later mentioned by the 2nd-century CE author Festus. The supposed location of this sacred grove is shared with the site of the Matralia, a woman-only festival celebrated on 1 March. Nearby altars to Mala Fortuna, the aspect of the goddess Fortuna associated with misfortune, and Febris, the goddess of fevers, may indicate that the air in this part of Rome was considered unwholesome. The historian Agata Szylińczuk suggests that the proposed location of this Esquiline sanctuary would have been situated near the Temple of Juno Lucina—the goddess of childbirth—and also near the temple of Venus Libitina, a chthonic deity. According to the classicist Gianluca de Martino, it is possible that this supposed sacred tree-grove could reflect a potential role of Mefitis as a fertility goddess.

In the 4th-century BCE, a sanctuary dedicated to the goddess was constructed at Rossano di Vaglio. Reconstructions of the settlement and the sanctuary are in the Museo delle Antiche Genti. Finds from this site include a set of 58 inscriptions, which were generally written in the Oscan language from the 4th-2nd century BCE. Afterwards, perhaps as a consequence of the Social War, the predominant language of the texts switched to Latin. These inscriptions mention the names of other deities, such as Hercules or Mamers, which are typically accompanied by epithets referring to Mefitis. According to the archaeologist Ilaria Battiloro, the exact function of this onomastic formula is unclear, though she suggests that these other deities may be cast in a subordinate role to Mefitis. Uniquely, the god Jupiter—who is mentioned several times in inscriptions from Rossano—is never paired with an epithet relating to Mefitis. Excavations at Rossano have uncovered a bronze plaque depicting a female deity riding a dolphin, which may connect to similar dolphin plaques discovered in Monte Pallano.

At Rossano, Mefitis herself is only paired with three epithets: Utiana, Caporoinna, and Aravinna. Etymologically, the name Aravinna may relate to the Latin term arva ("soil") and the title Caporoinna may connect to Latin caper ("goat") and the epithet Caprotina, which normally belongs to Juno. Semantically, the former title may imply that the goddess was associated with agriculture, and the second epithet may reflect possible fertility connotations. In Pompeii and Grumentum, Mefitis is attested with the epithet Fisica, an epithet of obscure origin. It is perhaps possible that this title is a transliteration of Ancient Greek φυσικά ("phusiká"), although it has alternatively been connected to Latin fidēs ("faith") and Oscan fisios. These two Italic terms may both imply a relationship with mediation, itself possibly related to the proposed divine role of Mefitis as a mediator between the heavens and the underworld. Mefitis may also have been associated with the epithet "διοϝιιας διομανα[σ] ("diowiias diomana[s]," "domina Iovia")" in one inscription. It is presumed that the text describes Mefitis as it was uncovered near a large altar by the sanctuary, which may imply that it was dedicated to the chief deity at this religious site, which was Mefitis.