Rubber-ball baseball (Japanese: 軟式野球, romanized: Nanshiki yakyū, lit. 'soft baseball') is a variant of baseball that originated in Japan and uses a rubber ball instead of a standard leather baseball. In English, it is also referred to as nankyu baseball, nanshiki baseball, or rubber baseball.

In a narrow sense, the term refers to baseball played with a hollow rubber ball; in a broader sense, it may also include related forms in which the surface of a hard baseball is replaced with rubber. By contrast, the ball used in softball is typically leather-covered but larger than a standard baseball.

Apart from the different ball, the rules are largely the same as those of conventional baseball. However, the properties of the rubber ball affect playing style, equipment, and technique.^[1]

The sport is widely played in Japan by both children and adults.

During the Meiji era (1868–1912), baseball introduced from the United States became established in Japan, particularly in schools and universities. Children also played informal baseball using hard tennis balls, but these proved unsuitable because of durability and speed.

The sport originated in Kyoto, where educators and members of a youth baseball research group developed a safer form of baseball using rubber balls.^[2]

In 1919, a rubber baseball was developed, and a tournament using the new ball was held at Kyoto Municipal Seitoku Elementary School, marking one of the earliest organized competitions in the sport.^[3]

A monument at Takaragaike Park Youth Sports Park (宝が池公園少年スポーツ広場, Takaragaike kōen shōnen supōtsu hiroba) in Kyoto commemorates the area as the birthplace of rubber-ball baseball.

The introduction of rubber balls contributed to the spread of youth baseball across Japan, leading to the establishment of organized associations and national competitions.^[3]

The development of rubber-ball baseball has been examined in academic research, which highlights the role of early 20th-century educators and local initiatives in Kyoto.^[4][5]

After the Second World War, the sport was reorganized nationally, and the Japan Rubber Baseball Association (全日本軟式野球連盟) was established in 1946.^[6] National competitions subsequently expanded, including major tournaments such as the All-Japan Rubber Baseball Tournament.^[6]

Rubber-ball baseball continues to be widely played at the youth level in Japan, with major national tournaments supported by local governments.^[7]

The ball has undergone several revisions, with current specifications established in December 2016 by the Japan Rubber Baseball Association (公益財団法人全日本軟式野球連盟) and the Baseball Ball Manufacturers Association (野球ボール工業会).

Two types are specified: type M (major) and type J (junior). Type M has a diameter of 71.5–72.5 millimetres (2.81–2.85 in), while type J has a diameter of 68.5–69.5 millimetres (2.70–2.74 in).^[8]

In comparison, a regulation leather baseball is 73–75 millimetres (2.9–3.0 in) in diameter, while an 11-inch softball has a diameter of about 89 millimetres (3.5 in).^[9]

• 👁 Rubber ball (left) and leather baseball (right)
  
  Rubber ball (left) and leather baseball (right)
• 👁 A K-ball designed to bridge rubber-ball and hardball baseball
  
  A K-ball designed to bridge rubber-ball and hardball baseball

While both metal and wooden bats are used in standard baseball, rubber-ball baseball typically uses metal or carbon bats designed for rubber balls.

Major competitions in Japan include:

General public

• All Japan Baseball Tournament (Men's Tournament)
• Emperor's Cup All-Japan Baseball Tournament
• Takamatsu-no-miya Cup All-Japan Baseball Tournament (Division 1, Division 2)
• All-Japan Women's Nanshiki Championship
• National Sports Festival (Men's Competition)

College students

• All Japan University Baseball Championship (Spring)
• All Japan University Women's Nanshiki Baseball Championship

Vocational school students

• All Japan Vocational School Nanshiki Championship

High school students

• All Japan High School Baseball Championship

In Cuba, a similar game known as cuatro esquinas is played, and related forms of rubber-ball baseball are also played in Taiwan.

Rubber-ball baseball is commonly played at the university level.^[10] The World University Rubber-ball Baseball Tournament (Japanese: 世界大学軟式野球大会, romanized: Sekai daigaku nanshiki yakyū taikai) is held periodically in Japan and overseas.^[11]

The International Boys Nankyu Baseball World Championship (Japanese: 少年軟式野球世界大会, romanized: Shōnen nanshiki yakyū sekai taikai), an annual international tournament for players under 12, is organized by the International Boys Nankyu Baseball Association Japan and held in Tokyo, typically at Edogawa Baseball Stadium.^[12][13][14]

• Japan Rubber Baseball Association (in Japanese)
• Tokyo Nanshiki Baseball Association (in Japanese)