Arruabarrena had few problems in her semi-final, needing just 57 minutes to beat Patricia Maria Tig 6-1 6-2 in Seoul on Saturday.
Lara Arruabarrena, Monica Niculescu into Seoul final
Lara Arruabarrena reached her first WTA Tour final in more than four years as Monica Niculescu joined her in the Korea Open decider.
The Spanish world number 90 was clinical, converting all five of her break points.
With her win, Arruabarrena reached a WTA Tour final for the first time since February 2012, when she clinched the Bogota title.
Niculescu booked her spot with a 6-0 6-4 triumph over Chinese third seed Zhang Shuai.
The Romanian fifth seed cruised early before battling through the second set, which featured seven breaks of serve.
Niculescu will be eyeing her third WTA Tour title when she meets Arruabarrena.
The pair have met twice before, winning once each.
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