Japan, who scored 24 goals in their first four matches, penned their opponents back for much of the game and created plenty of scoring opportunities in the semi-final tie at Sydney's Stadium Australia.
The Nadeshiko went ahead in the 15th minute when midfielder Fuka Nagano stole the ball on the edge of South Korea's penalty area and squared for Riko Ueki, who made no mistake from close range as she netted her fifth goal of the tournament.
They doubled their lead 10 minutes later as Maika Hamano wriggled past two South Korean defenders, before unleashing a vicious shot from a tight angle which squeezed in at the near post.
Defender Saki Kumagai met Momoko Tanikawa's corner with a powerful header to make it 3-0 in the 75th minute, and while South Korea clawed a goal back through Kang Chae-rim, Remina Chiba netted Japan's fourth in the 81st minute to snuff out any comeback hopes.
Kang's goal was the first that Japan conceded in the tournament.
"I was very proud of them today. Sometimes it's not easy to just switch and play (a team) that has much more to give and make it look easy - and they actually did it," Japan coach Nils Nielsen told Australia's Channel 10 TV.
"They actually made this game look fairly easy and comfortable for us but it wasn't, because South Korea is a great opponent."
Japan take on Australia in the final at the same venue on Saturday, after the hosts defeated nine-time champions China 2-1 on Tuesday.
"The Matildas really have an amazing team, they have adapted to whatever is coming their way," the Danish coach added.
Japan are looking to win a third Asian Cup title, after also lifting the trophy in back-to-back editions in 2014 and 2018, defeating Australia in both of those finals.
Australia have won the tournament once since returning to the Asian Football Confederation in 2006.