The true identity of the Sumiyoshi Sanjin is the three kami of purification belonging to Izanagi-no-Mikoto, appearing in the first volume of the Kojiki (Age of the Gods).^[1] When Izanagi returned from Yomi (the underworld) and performed misogi (purification) at Ahagihara in Himuka of Tsukushi, he submerged himself in seawater to cleanse his body. From three different depths, three deities were born: recorded as 'Sokotsutsu-no-o-no-kami, Nakatsutsu-no-o-no-kami, Uwatsutsu-no-o-no-kami' in the Kojiki, and as 'Sokotsutsu-no-o-no-Mikoto, Nakatsutsu-no-o-no-Mikoto, Omotetsutsu-no-o-no-Mikoto' in the Nihon Shoki. The orthographic difference between 'Uwa' (upper) in the Kojiki and 'Omote' (surface) in the Shoki is one basis for the later interpretation of 'tsutsu' as layers of water. Simultaneously, the three Watatsumi kami (Sokotsu, Nakatsu, Uwatsu Watatsumi) were born, making Sumiyoshi and Watatsumi a paired dual structure—bottom = Sokotsutsu/Sokotsu Watatsumi; middle = Nakatsutsu/Nakatsu Watatsumi; surface = Uwatsutsu/Uwatsu Watatsumi. This tripartite structure is common to both texts.

The etymology of 'Tsutsu' has not been academically resolved. Major theories are listed alongside each other:^[2] ① Star theory — 'Tsutsu' is an archaic word for 'star' (hoshi); it deifies the three central stars of Orion (ancient name 'Miboshi') as navigational stars for ancient seafaring clans. However, this is a modern theory advocated by Hoei Nojiri in "Japanese Stars" (1936), and there are no direct supporting primary texts from early folklorists like Orikuchi or Yanagita; treating it as the 'accepted folkloric theory' is academically inaccurate. ② Tsu (port) theory — 'Tsu' is a particle meaning 'of', and the second 'tsu' means 'port/sea route', an interpretation from Orikuchi's lineage; ③ Tsuchi phonetic change theory — 'Tsu' is a particle, and 'chi' is an honorific spirit suffix (akin to Orochi or Nozuchi); ④ Tsutsu-ro theory — 'Tsuchi' equals 'tsuji' meaning sea route; ⑤ Ship spirit theory — ancient ship spirit worship enshrined at the bottom of a boat; ⑥ Tsutsu toponym theory — derived from Tsutsu at the southern tip of Tsushima, the origin of seafaring clans; ⑦ Literal tube theory — using bamboo tubes as a yorishiro (vessel). Listing multiple theories is academically accurate, and it is particularly imprecise to refer solely to the 'star theory' as the accepted view.

The Empress Jingu legend is the most important narrative in the history of Sumiyoshi Sanjin worship.^[3] According to the Nihon Shoki, when Empress Jingu became possessed after Emperor Chuai's death, the Sumiyoshi kami delivered an oracle: "Conquer Silla, a land filled with gold and silver. If you worship us three deities, Silla and Kumaso will submit." They protected her maritime expedition (subjugating Silla, Baekje, and Goguryeo), and upon her return, a second oracle commanded: "Enshrine our Aramitama (rough spirit) in Yamada village of Anato (Nagato)" — this became the origin of the Shimonoseki Sumiyoshi Shrine. Enshrining the Nigimitama (gentle spirit) in Settsu became the origin of Sumiyoshi Taisha. The structure of jointly enshrining Empress Jingu and Sumiyoshi Sanjin originated here, establishing the unique four-hall structure of Sumiyoshi Taisha. However, the dating of the Empress Jingu chronicles is heavily debated by scholars, and treating the legendary date (211 AD) as historical fact requires extreme caution—archaeologically, it is pointed out as potentially reflecting events from the 4th century onwards.

Sumiyoshi Taisha, the head shrine (2-9-89 Sumiyoshi, Sumiyoshi-ku, Osaka), is the Ichinomiya of Settsu Province, one of the Twenty-Two Shrines (Middle Seven Shrines), and a former Kanpei-taisha.^[4] Its official history dates its founding to the 11th year of Empress Jingu's regency (211 AD) on the first Day of the Hare in the Month of the Hare — a legendary date without archaeological proof. Its four main halls have a unique layout: the First, Second, and Third Halls line up vertically (facing west, toward the sea), while the Fourth Hall sits south of the Third, forming an L-shape. The First Hall enshrines Sokotsutsu-no-o, the Second Nakatsutsu-no-o, the Third Uwatsutsu-no-o, and the Fourth Empress Jingu. "Sumiyoshi-zukuri" is considered the oldest style in shrine architectural history, characterized by a gabled roof, cypress bark thatch, and red and white walls. The current main halls were built in 1810 and are all designated National Treasures. The steeply arched red Sorihashi (Taiko Bridge) is the iconic visual symbol of the Sumiyoshi faith, frequently appearing in ukiyo-e, paintings, and waka poetry.

There are over 2,300 branch shrines nationwide (the official number from Sumiyoshi Taisha, whereas Wikipedia undercounts at ~600). The distribution is concentrated along coastlines, ports, the Seto Inland Sea, Kyushu, and northern Japan, proving it to be the most vital faith for fishermen, shipping merchants, and navy personnel from ancient to modern times.

The "Three Great Sumiyoshi" and ancient shrine dispute: ① Sumiyoshi Taisha (Osaka) = Settsu Ichinomiya, Nigimitama, Head Shrine; ② Sumiyoshi Shrine (Shimonoseki, Yamaguchi) = Nagato Ichinomiya, Aramitama, site of Empress Jingu's return oracle; ③ Sumiyoshi Shrine (Hakata, Fukuoka) = Chikuzen Ichinomiya, self-proclaimed "Japan's First Sumiyoshi-gu", oldest theory identified with Ahagihara. Additionally, Hon-Sumiyoshi Shrine in Kobe is based on an influential Edo-period theory by Motoori Norinaga (1764-1798) identifying Settsu's Sumiyoshi village as the ancient "Nanakura's Long Strait". Academically, the "first Sumiyoshi" cannot be determined, with each shrine claiming primacy based on its own legends.

In ancient and medieval times, it was customary for Japanese envoys to Sui and Tang China to pray at Sumiyoshi Taisha before departing. The "Tosa Nikki" (Ki no Tsurayuki, 935) also notes prayers for maritime safety to the Sumiyoshi deities. In Heian-period waka by poets like Izumi Shikibu, Ki no Tsurayuki, and Ono no Komachi, Sumiyoshi frequently appears, positioning the kami at the head of the "Three Deities of Waka". In the medieval and early modern periods, the Noh play "Takasago" featured the "Pines of Sumiyoshi and Takasago" as a symbol of marital harmony and longevity, often performed at shrine weddings. The Noh play "Sumiyoshi Mode" is another masterpiece representing this faith. The rice-planting ritual (Otaue Shinji), a National Important Intangible Folk Cultural Property, is Sumiyoshi Taisha's most representative festival, sanctifying the entire rice-farming cycle from planting to harvest.

As a samurai faith from the medieval to Edo periods, they garnered reverence from warrior clans like the Minamoto due to Empress Jingu's Sankan Seibatsu legends. During the Muromachi and Sengoku periods, Sumiyoshi Taisha received immense veneration from the shipping industry in the Seto Inland Sea, Settsu, and Izumi, actively participating in both commercial and military affairs as the guardian of maritime traffic in Osaka Bay.

Today, visits by the Maritime Self-Defense Force, merchant ships, fishermen, and shipping businesses remain robust. It is one of Osaka's premier spots for New Year's visits, Shichi-Go-San, and shrine weddings. Affectionately called "Sumiyoshi-san" in the Kansai region, the Sumiyoshi Sanjin are a national divine presence offering wide-ranging blessings for maritime protection, safety at sea, poetry, scholarship, marital harmony, safe childbirth, and business prosperity. The 2,300 branch shrines, known as Sumiyoshi Shrine, Sumiyoshi-sha, Sumie Shrine, or Sumiyoshi Jinja, line Japan's coastlines and ports, forming the backbone of an uninterrupted maritime faith from ancient times to the present day.