The digital bank did not offer an explanation about Lee's decision to step down. Lee became K bank's chief in April 2020 and his term was set to last until March 2022.
The resignation came as a surprise given the efforts Lee made to normalize operations at the firm.
K bank, the first internet bank established in Korea in 2017, faced obstacles in securing capital in its early years. This resulted in a virtual suspension of operations in 2019.
New laws went into effect last year that enabled K bank to get back on its feet with fresh capital injections.
Lee is known to have made all-out efforts in this process, pitching to shareholders last year and sharing the digital bank's vision for the future.
The online-only bank appeared to be back on track in 2020 with new investments and new loan services drawing attention.
Expectations had grown for the lender because Lee was formerly CEO of BC Card and had also worked as a senior executive with the telecom giant KT. Lee's background and ties raised expectations that the digital bank would experiment with and develop innovative fintech services.
K bank has yet to catch up with its rival Kakao Bank, which has been growing fast. Kakao Bank achieved its first annual surplus in 2019 and aims to go public this year.
K bank faces an increasingly tough business environment given the financial services platform Toss is also set to launch a digital bank this year. The next CEO will have to assume the heavy burden of securing new investments and stabilizing the lender.
While BC Card has become the largest shareholder of K bank, KT is the lender's de facto controller. BC Card is an affiliate of KT. K bank's first CEO Shim Sung-hoon, as well as Lee, were with KT.
Some were expecting K bank to scout experienced personnel from outside for the next CEO, as the lender is in need of a digital banking expert. But given KT's long-standing preference to give top authority to insiders, it's less likely for K bank to name an outside expert as its next chief. K bank's Executive Vice President Chung Woon-ki is serving as interim CEO.