The 36-year-old hooker had been due to hang up his boots after the World Cup, but said he intended to "have fun" when the new Japan Rugby League One campaign kicks off on Saturday.
"I'm glad I've taken it, it's only an extra six months," he said, after being introduced to reporters alongside another new Kubota signing, Welsh full-back Liam Williams.
"I'm going to give everything I've got and have a bit of fun with it."
Kubota lost their South African hooker Malcolm Marx for the season with a knee injury suffered at the World Cup.
They then turned to Coles, who had originally announced in February that he would retire after the tournament in France.
"I've only been here a week and I feel like, maybe not a 21-year-old, but I feel young and energised by the group," said Coles, who won 90 Test caps for the All Blacks.
"It's completely different to New Zealand but it's a lot of energy, a lot of laughter and a lot of hard work, which is something I've really enjoyed so far."
Kubota head coach Frans Ludeke, who has been linked with the still-vacant Japan national team job, said signing Coles was "a big blessing" for his team.
"Dane changed the image of hookers," he said of the dynamic front-row forward who scored 23 tries for New Zealand.
"Normally, hookers were big at the scrums and set-pieces but the way he plays is like a loose forward.
"He's got an amazing skill set."
