Kim Kwan-Jin, who has been the South's defence chief since 2010, will take up the security brief with Han Min-Koo filling Kim's vacated post, Park's spokesman said.
Han was the chairman of the Joint Chiefs of Staff from 2010 to 2011 and had previously served as the chief representative for general-level military talks between the two Koreas.
"We believe Han -- well respected within the ranks and who has great insights in both field situations and policies -- is the right person to strengthen our national defence," Min Kyung-Wook said.
Kim's predecessor, Kim Jang-Soo, quit after a public outcry over his comments that his office was not responsible for handling the Sewol ferry accident that killed around 300 people.
The Sewol sank off the country's southwest on April 16, leaving 288 passengers -- mostly teenagers on a school trip -- dead and 16 still unaccounted for.
Park's government has faced strong criticism over botched rescue efforts and lax safety by ferry operators.
The prime minister also resigned during the fallout.
The naming of the defence chief also comes at a time of elevated tensions between the two Koreas.
The North last week warned that recent "provocative" activities by US troops at a truce village on the tense inter-Korea border could lead to a "catastrophic" military clash.
Last month, Seoul said a North Korean warship fired shells near one of its naval ships and denounced Pyongyang's denial as a "blatant lie."