By tournament:
1959 YC & AC Sevens (Yokahama Sevens)
1993 Tokyo Sevens
The list may be subject to change if more rugby union sevens tournaments are discovered.
Japan’s introduction to rugby union
Scotland had an organised football club in 1824 (simply known as the Foot Ball Club), but it played a hybrid association-rugby type game. The initial rules of the rugby code came to Scotland by the 1850s; the Story of Scottish Rugby by R.J. Phillips stating that it was in Scotland by 1851; with Edinburgh Academicals being formed in 1858. (That rugby code was later modified by The Green Book; the Blairgowrie and Rattray Laws; and the Kilmarnock Rules; into the rugby union recognisable today.)
Only a few years later, rugby union came to Japan in the 1860s, introduced by Scots. There is a possibility of the game being played in 1863, but it is firmly being played in Yokohama by 1866.
Scotland is credited with introducing rugby to Japan. For many it was believed that rugby was first played in Japan in 1899. However, research by Japan rugby historian Mike Galbraith revealed that two Scots, Lord Walter Kerr, who was to become Britain’s senior naval officer, and George Hamilton, who was educated at Rugby School in England, had played pivotal roles in bringing rugby to the country in the 1860s at Yokohama Football Club.
https://news.yahoo.com/whisky-bagpipes-opera-scots-japan-201644338–spt.html
For the 1863 possibility: there is a note in a 1908 Australian newspaper in which Harry Rawson, Governor of New South Wales, “recalled playing in the first cricket match played in Japan in 1863, a remarkable feature of which was the fact that half the players were playing football.” However it is not known whether the football played was association or rugby union.
Even this 1863 cricket match – with some players playing football – is Scottish influenced. That first Japanese cricket match heavily features James Campbell Fraser (1840-1913), a Scottish insurance merchant in Japan.
However the rugby union history of Japan is more concerned with his brother Evan James Fraser, and his Scots friends George Hamilton and James Mollinson; and the Mollison Shokai building (No. 48) where Scots concentrated and where they worked.
For Japan’s rugby union history we are on much firmer ground with the establishment of Yokohama Football Club on 26 January 1866. It was James Mollinson who founded Japan’s first rugby club; and George Hamilton became its captain.
“More than forty names have been put down as willing to support a Foot ball Club,” says an editorial in The Japan Times, and “as we happen to have two or three Rugby and Winchester men in the Community, that we may be certain that we shall have really good scientific play.”
https://galbraith.press/1866-and-all-that-the-untold-early-history-of-rugby-in-japan-the-japan-times-2/
From Britain and Japan by Mike Galbraith:
Yokohama. The Building no. 48
Kanagawa Prefecture’s oldest surviving Western structure is the remnant of the ruined building, currently named Mollison Shokai or Mollison & Co., located at the corner of a block of land now called No. 54, which absorbed No. 48. It is a Kanagawa prefectural cultural asset with protected status. The Mollison in the Mollison Shokai name was James Pender Mollison who lived in No. 48 for many years from 1868. There used to be two buildings at No. 48; what remains is part of the office but adjacent to it was a residence. The two buildings were the home and work place of a group of prominent Scottish businessmen who also played an important role in the sports scene in Yokohama for most of the second half of the nineteenth century.
https://www.degruyter.com/document/doi/10.1515/9781898823469-011/html?lang=en
Unsurprisingly the Mollison Shokai building, the workplace of Scots in Yokohama, saw Scots bringing their sports to Japan. Galbraith notes 3 such Scots below:- James Mollison, George Hamilton and Evan James Fraser.
In 1867 or very early 1868, George Hamilton arrived. Hamilton was Scottish and had studied at Rugby School, the famous English public school where, in 1823, William Webb Ellis supposedly first ran forward carrying the ball, thus creating the sport of rugby.
James Mollison, who founded the Yokohama Cricket Club in mid-1868 together with Hamilton, described Hamilton as “captain of the Yokohama Rugby Team.” Hamilton worked in the same office as Mollison and, in around 1870, Evan James Fraser, another Rugby School alumni from Scotland, arrived to manage the same office. Fraser, like Hamilton, was an excellent all-round sportsman.
James Pender Mollison (born 21 July 1844, Glasgow – died 22 November 1931, Kamakura), born to William Mollison and Margaret Pender. He married Isabella Duff.
George Hamilton (born 24 November 1844, Glasgow – died 15 September 1929, Manhattan) born to William Hamilton (1806-1866) and Margaret Buchanan Henderson (1819-1850). He married Bertha Torrance (1861-1963) in 1885 in New Jersey, USA. They had 2 girls Amy and Peg; and 2 boys Minard and Kenneth.
https://www.familysearch.org/tree/person/details/KZBK-4VN
In 1870, Mollison and Hamilton were joined by Evan James Fraser.
Evan James Fraser (9 April 1847, Skipness, Argyll – 8 November 1911, Manhattan) [also known as Evan James Fraser-Campbell; his mother was a Colquhoun Campbell and he changed his surname by deed poll in the USA in 1889 to obtain Dunmore.] He came to Japan to work with his brother James Campbell Fraser. His brother ran J. C. Fraser & Company, insurance merchants. Evan moved to New York in 1878, marrying Edna Arnold in 1881 and they had 3 sons. He was the proprietor of Dunmore in Argyll.
TWO SCOTTISH ‘CASTLES’ IN KINTYRE WHERE TWO KEY FIGURES IN EARLY HISTORY OF SPORT
This text is from Bernd Lepach’s Meiji Portraits:
[Evan James Fraser] came to Japan in 1870 and was employed by J. C. Fraser & Co., Insurance Merchants, Yokohama # 48. He worked with this firm until 1876, from 1874 as Manager, when J. C. Fraser retired. In 1875 this firm additionally included the tea export with tea-firing. In 1877 he established in partnership with James P. Mollison the firm Mollison, Fraser & Co., Yokohama # 48. From the very beginning James P. Mollison managed the firm in Japan and Evan J. Fraser stayed in GB. He never returned to Japan, in 1883 he sold his shares to J. P. Mollison, who, from 1884 on, operated under Mollison & Co., Yokohama # 48.
[George Hamilton] came to Japan around 1870 on behalf of J. C. Fraser & Co. and was employed in the Yokohama branch # 48. He worked for the company until early 1875, when he managed to secure a contract with the Japanese government, Ministry of Public Works, to work as a teacher at the preparatory school of the engineering school. His contract began on July 1, 1875 and ended on April 30, 1877.
In 1876, JC Fraser went out of business. His brother Evan James Fraser founded Mollison, Fraser & Co. in partnership with James Mollison in 1877 at Yokohama # 48, and George Hamilton worked for this company after his teaching position. In 1884 the partners separated and James Mollison continued to run the company under Mollison & Co., Yokohama # 48; George Hamilton became his partner. In 1885 he left Japan and settled in New York, where he worked for the company until 1892. After that, his traces are lost. George Hamilton was an active member of the Yokohama Cricket & Athletic Club (YC & AC)
in the 1870s. He not only worked as a secretary for many years, but was in particular an active, all-round sportsman, especially in rugby, cricket, football, rowing, etc.
http://meiji-portraits.de/meiji_portraits_h.html
Yokohama Sevens
The Sevens tournament was first run by the Yokahama County and Athletic Club in 1959. This is the same club founded by James Mollinson in 1866. Since 2012 it has been also sponsored by the Japan Rugby Union.
Tokyo Sevens
The Tokyo Sevens tournament began in 1993. Meiji University won the tournament in 1993, and Suntory won it in 1994. It became an international tournament in 1995 with Fiji running out winners . It is also known as the Japan Sevens.