"I have seriously considered and thought about how I can put my experience and knowledge as a UN Secretary General to best use," Ban said in televised remarks after his arrival.
Speaking before a crowd of hundreds at Incheon International Airport, Ban said he was prepared to meet with the public this week before making a "selfless decision".
Ban's return comes at a time of national uncertainty with tens of thousands of people marching in weekly protests demanding the permanent removal of scandal-hit Park, who was impeached by parliament last month over a corruption scandal.
"It breaks my heart," said Ban, referring to the ongoing political turmoil.
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"We must never forget the people's desire that was expressed on the streets."
A career diplomat, Ban never joined any South Korean political party, although he served as foreign minister under the late liberal president Roh Moo-Hyun from 2004 to 2006.
It remains unclear whether Ban will join Park's ruling Saenuri Party or the emerging conservative party.
Saenuri Party floor leader Chung Woo-Taik "sincerely" welcomed Ban's return through a Facebook post on Thursday, calling him "the pride and national asset of South Korea".
Further troubling his political aspirations, Ban's younger brother and nephew were indicted in a US court this week over an alleged attempt to bribe a Middle Eastern official regarding the sale of a building in Vietnam.
However Ban has denied any involvement, saying he was "perplexed and embarrassed" by the situation.
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