The wife of former South Korean president Yoon Suk-yeol interfered in state matters in return for expensive valuables and money, a special prosecutor said on Monday.

The special prosecutor's investigations, which wrapped up on Sunday, came amid a year-long probe into Yoon's brief imposition of martial law last year and related scandals linked to the once-powerful couple.

The prosecution team this month sought a 15-year prison sentence for former first lady Kim Keon-hee, who is under detention and currently on trial for suspected acceptance of bribes for mediation and other charges.

Kim ⁠has denied any wrongdoing.

She apologised to the public for causing concerns during a court hearing earlier this month.

Kim "took advantage of the status of the president's spouse to receive money and expensive valuables and had been widely involved in various personnel appointments and nominations," special prosecutor Min Joong-ki said at a news conference marking the end of his investigation.

Kim is alleged to have received bribes from businesses and politicians amounting to US$263,000.

Investigators alleged Kim had received two Chanel bags and a Graff necklace from the leader of the Unification Church.

She also allegedly received luxury jewellery, a painting by famed South Korean minimalist painter Lee Ufan, a Dior handbag and a wristwatch.

The former first lady has denied all the charges.

In her final testimony, she said the allegations were "deeply unjust".

"Yet when I consider my role and the responsibilities entrusted to me, it seems clear that I have made many mistakes," she said.

A lower court ruling on Kim is expected on January 28.

"Investigations do not end because one says so but are eventually completed with evidence in court," Kim's lawyers said in a statement on Monday, adding they would work "to ensure that procedural legitimacy and defence rights are thoroughly guaranteed so that facts are not exaggerated or distorted into political framing".

The prosecution team has also indicted Unification Church leader Han Hak-ja, ⁠now on trial, after the religious group was suspected of giving Kim valuables including two Chanel bags and a diamond necklace as part of its efforts to win influence.

Han has denied that she directed her church to bribe Kim.

"Various people who did not have a common denominator with each other visited Kim Keon-hee, not the president, and asked for what they wanted, and gave money and goods," said assistant special prosecutor Kim Hyeong-geun.

"As a result, their request was realised."

Yoon is on trial for suspected masterminding of an insurrection, a charge that could mean a life sentence or even a death penalty. He has denied the charges.

A ⁠lower court ruling on Yoon is expected in early 2026. (Reuters/AFP)