"We have to look into issues including the possibility of sending back THAAD, if it has not properly undergone domestic legal procedures," Yonhap news agency quoted Woo as saying during a radio interview.
The parliamentarian referred to the lack of parliamentary approval of this crucial national security decision.
The statement comes after a statement made by US President Donald Trump in late April, in which he informed Seoul that "it would be appropriate if they paid" for the deployment of a US THAAD system on South Korean soil.
In April, media reported, citing a US military official, that the THAAD system in South Korea would be operational in the very near future.
In July 2016, Washington and Seoul reached an agreement on placing a US THAAD system in South Korean territory. In early March the THAAD deployment began in response to North Korea’s ballistic missile tests. The move provoked sharp criticism by a number of countries, including China, which claimed the installment of the missile defense system would undermine its security.