Defense Ministry spokesman Kim Min-seok says a navy ship carrying what appeared to be a fuel tank arrived Friday at a military base in Pyeongtaek.
The tank was inscribed with the name of the "Unha-3" rocket.
Kim said the debris will be key to determining the true content of the rocket.
South Korea's Yonhap news agency reported the navy vessel recovered the object in the Yellow Sea near the rocket's trajectory announced by the North.
"We will be able to find out the metal component and its size from the debris," said Kim. "If we know the size,we will be able to estimate the amount of its content. If there is residual component inside the tank [of debris, which was recovered] we will be able to calculate what kind of energy the component turns into. Thus, we are expecting to find out things such as thrust force "
North Korea has celebrated the rocket launch as a step toward peaceful exploration of space, saying its goal was to put a weather satellite into orbit. But South Korean Unification Minister Yu Woo-ik believes the launch was a test to acquire nuclear-related technology.
"The purpose of this missile test is to acquire the technology required to transport a nuclear warhead and nuclear weapon, so there is a high probability a nuclear test might follow it," said Yu.
The launch Wednesday has drawn the ire of the international community. The U.N. Security Council condemned it as a violation of resolutions barring North Korea from carrying out missile or nuclear tests.
But North Korean leader Kim Jong Un vowed Friday to send more satellites to space to develop the country's "science, technology and economy."
Analysts have acknowledged the object placed into space does appear to have achieved orbit, but have not yet confirmed whether it is successfully communicating with Pyongyang.
The U.N. Security Council has imposed two rounds of sanctions against North Korea, following nuclear tests in 2006 and 2009. After a failed North Korean rocket launch in April, the council ordered foreign assets seized from several North Korean companies linked to financing and procuring weapons and missile technology.