There had been speculation about his future after a rollercoaster performance by Japan, who again failed to reach the quarter-finals but produced stunning wins against Germany and Spain.
"This is really the happiest job", Moriyasu said of his reappointment.
"It's a job that gives me a chance to compete on a global stage with the pride of being Japanese. That's why I decided to accept the offer", he said.
It was the fourth time Japan had exited at the first knock-out stage and denied them a much-coveted quarter-final debut in their seventh straight World Cup appearance.
Moriyasu becomes the first Japan coach to stay on after a World Cup.
The 54-year-old's new contract will run "until the next World Cup", said Japan Football Association head Kozo Tajima, who lauded Moriyasu for "elevating the international status of Japan's football".
After the heartbreak of their exit in Qatar, Moriyasu insisted Japan are on the right path.
"We weren't able to overcome this hurdle of losing in the last 16 and you might say that we didn't achieve anything new," he said.
"But the players have shown us something that we haven't seen before by beating former champions like Germany and Spain."