Dental calculus, or calcified plaque, is a potential source of archaeological information relating to the lives of past societies. The recovery and identification of organic microremains entrapped within dental calculus provide new data regarding diet, health, and lifestyle. This paper presents, for the first time, microscopic evidence of the sporangium annulus of a fern within the dental calculus of an adult male from the medieval necropolis of Can Reiners (Balearic Islands, Spain). The sporangium evidence was compared with modern sporangia collected in northern Mallorca and the Iberian Peninsula. The results revealed that the features of the Can Reiners sporangium sample were compatible with the species Asplenium trichomanes. Through a historical and modern botanical literature review, it is known that this species has been widely regarded as a cure for kidney stones and alopecia. Moreover, this species was used as an expectorant, diuretic, and emmenagogue (herb that provokes menstruation or abortion). Considering that sporangia are attached to leaves and when they are mature only spores are released in the air, the presence of the annulus within dental calculus provides direct evidence of the consumption of ferns by the population living in the Balearic Islands during the Middle Ages. Although the use of these plants as constituents of diet is not attested, we may suggest that they could be employed as medicinal plants.