In the Celtic polytheism of classical antiquity, Grannus (also Granus, Mogounus, and Amarcolitanus) was a deity associated with spas, healing thermal and mineral springs, and the sun. He was regularly identified with Apollo as Apollo Grannus. He was frequently worshipped in conjunction with Sirona, and sometimes with Mars and other deities.
In the early twentieth century, the name was connected with the Irish grian, ‘sun’. Along these lines, the god was often linked to the Deò-ghrèine and the character Mac Gréine of Irish mythology. However, the Irish grian, ‘sun’ is thought to be derived from Proto-Celtic *greinā ‘sun’ and the Proto-Celtic *greinā is unlikely to have developed into Grannos in Gaulish and other Continental Celtic languages. Derivation from a Proto-Celtic root *granno- ‘beard’ (cf. Middle Welsh grann ‘chin; beard, hairs’ and Old Irish grend ‘beard, hairs’) has enjoyed some scholarly support, from which Jürgen Zeidler dissents, proposing a different root *granno- with "probable reference to the sun's heat and healing properties". Ranko Matasović, in his Etymological Dictionary of Proto-Celtic, has tentatively proposed that the root of this theonym comes from Proto-Celtic *gwrenso-, which means "heat"; however, Proto-Celtic initial gw- yields b- in Gaulish (cf. the name Brennus).
In all of his centres of worship where he is assimilated to a Roman god, Grannus was identified with Apollo, presumably in Apollo’s role as a healing or solar deity. In Trier, he is identified more specifically with Phoebus as Apollo Grannus Phoebus.
One of the god’s most famous cult centres was at Aquae Granni (now Aachen, Germany). Aachen means ‘water’ in Old High German, a calque of the Roman name of "Aquae Granni". The town’s hot springs with temperatures between 45 °C and 75 °C lay in the somewhat inhospitably marshy area around Aachen's basin-shaped valley region. Aachen first became a curative centre in Hallstatt times.
According to Cassius Dio, the Roman Emperor Caracalla (188 AD to 217 AD) unsuccessfully sought help from Apollo Grannus—as well as Aesculapius and Serapis—during a bout of physical and mental illness, visiting the god's shrine and making many votive offerings; Dio claims that the gods refused to heal him because they knew Caracalla's intentions to be evil. Caracalla's visit to the shrine of ‘the Celtic healing-god’ Grannus was during the war with Germany in about 215.
In the early twentieth century, the god was said to have still been remembered in a chant sung round bonfires in Auvergne, in which a grain sheaf is set on fire, and called Granno mio, while the people sing, “Granno, my friend; Granno, my father; Granno, my mother”. However, granno may simply be a derivative of an Occitan word of Latin origin meaning "grain" (compare Auvergnat gran "grain", grana "seed" and Languedocien grano, from Latin grānum "grain").
A 1st century AD Latin inscription from a public fountain in Limoges mentions a Gaulish ten-night festival of Grannus (lightly Latinized as decamnoctiacis Granni):
The name Grannus is sometimes accompanied by those of other deities in the inscriptions. In Augsburg, he is found with both Diana and Sirona; he is again invoked with Sirona at Rome, Bitburg, Baumberg, Lauingen, and Sarmizegetusa (twice). At Ennetach he is with Nymphs, at Faimingen with Hygieia and the Mother of the Gods, and at Grand with Sol. A votive altar at Astorga invokes him after "holy Serapis" and "the many-named Isis", and before "the unvanquished Core and Mars Sagatus".