The strait is only 19.5km wide at its narrowest point, but Tominaga ended up swimming 38km.
A Japanese man aged 73 became the world's oldest person to swim across the country's Tsugaru Strait on Wednesday, supporters said, braving unpredictable currents and stinging jellyfish in his quest.
Toshio Tominaga's record bid saw him spend about 10 hours swimming across the channel in northern Japan that is considered one of the world's most difficult because of strong and unpredictable currents, his supporters said.
The strait is only 19.5 kilometres (12 miles) wide at its narrowest point, but Tominaga ended up swimming 38 kilometres, ploughing northward as he battled currents moving eastward, support team member Masayuki Moriya said.
Tominaga "is so exhausted that he needs to rest", Moriya told AFP minutes after the achievement.
Organisers had expected he would have to swim for about 12 hours, covering 45 kilometres, but currents were weaker than forecast for much of the journey, Moriya said.
"Until the last five kilometres it looked easy for him but the final leg was very hard with strong currents," he added.
Toshio Tominaga swims in his challenge. (AFP/ Ocean Navi)
The septuagenarian began his swim at 5:28 am Wednesday (2028 GMT Tuesday). Nine hours and 58 minutes later, he arrived on the other side of the strait, supporters announced on Facebook.
"He became the world's oldest" person to accomplish the feat, the post said.
Marathon swimmers usually attempt Tsugaru as part of their bid to complete the world's "Oceans Seven" long-distance swims, along with the English Channel, the Strait of Gibraltar and four others, according to the World Open Water Swimming Association.
In the past, swimmers have encountered sharks and fended off jellyfish and squid to complete the crossing, a feat first accomplished by US athlete David Yudovin in 1990.
In 2012 American woman Pat Gallant-Charette became the oldest person to swim across Tsugaru at 61.
Last month 18-year-old Japanese swimmer Honoka Hasegawa became the youngest person and the fastest woman to cross the strait.