Han, 75, will resume office and take over the official duties of suspended president Yoon Suk Yeol.
Seven of the court's eight justices on Monday overturned or dismissed Han's impeachment.
They ruled the accusations against him weren't against the law or weren't serious enough to remove him from office or his impeachment motion didn't even met a required quorum when it passed thorough the assembly three months ago.
Han, who was also serving as acting president, was accused of being involved in Yoon's short-lived declaration of martial law on December 3 that triggered a massive political crisis.
Han denied the allegations and lasted less than two weeks in the post before being impeached and suspended on December 27 after clashing with the opposition-led parliament by refusing to appoint three more justices to the Constitutional Court.
The unprecedented, successive impeachments that suspended the country's top two officials intensified a domestic division and deepened worries about the country's diplomatic and economic activities.
The deputy prime minister and finance minister, Choi Sang-mok, has been serving as acting president.
The Constitutional Court said it had decided to overturn Han's impeachment, but it has yet to rule on Yoon's impeachment.
Yoon's declaration of martial law was seen as an attempt to crack down on his political opponents.
It was short-lived but plunged South Korea into political turmoil, with massive rival rallies backing Yoon or denouncing Yoon dividing the streets of Seoul and other major cities in South Korea.
At the time, Yoon claimed the opposition had been infiltrated by communist forces, although no evidence supported the accusation.
The Constitutional Court is expected to rule soon on Yoon's impeachment case.
On March 8, Yoon was released from prison, after a Seoul district court allowed him to stand his criminal trial without being detained.
If Yoon's impeachment is upheld, new elections would be required within 60 days.
If not, Yoon would return to office.
While the judges are expected to issue their verdict as early as this week, no official date has been set.
The political crisis has severely damaged South Korea's reputation internationally and stalled the country's economic growth.
with Reuters and AP
28°C