One of the year's most spectacular startup payoffs isn't in Silicon Valley or China, it's in Japan.

Mercari Inc.'s $3.7 billion coming-out party will deliver a 149-times return to early investor United Inc., which engineered a ¥300 million ($2.7 million) round in the flea-market app in 2013. That stake will be worth at least ¥45 billion when Mercari goes public on Tuesday, a major windfall that may galvanize the No. 3 economy's stunted startup scene.

"The environment is right to see several more unicorns appear over the coming decade," said Yozo Kaneko, president of the Tokyo-listed investment and internet services firm. "But the real consequence is that we'll see more experienced entrepreneurs come out of those unicorns, and they're the ones who could have a really big impact on the entire ecosystem."