The man, identified only by the surname Han, scored 397.5 out of 400 in the first round, missing one question out of 160. He ranked first in both rounds of the qualification exam, held in April and July.
The admission comes days before Thursday's annual national college entrance exam, a high-pressure test for which many South Korean students spend much of their lives preparing. It has raised the stakes for the organisers of that exam.
The marks had seemed too good to be true. Months of speculation followed near-perfect results on hundreds of questions across the two tests, and sceptics were proved right when he confessed.
Han said he took photos of the exam questions with a microcamera, sent them to an AI tool and checked the answers on a smartphone. He had initially denied accusations that he cheated with smart glasses.
"My passing the exam with the top score was not the result of fair effort,"Han said as he apologised on the YouTube channel of Attorney Lee Don-ho.
He confessed through his lawyer that he used AI to obtain answers during the tests. The Seoul metropolitan police agency said it received a written confession through the lawyer that morning and assigned the case to investigators.
"We received a written confession through the test taker's lawyer this morning regarding cheating on the tax accountant exam,"the agency said.
A senior presidential office official told reporters on Sunday that the office viewed the matter very seriously."Because the college entrance exam is next week, we have been working hard to prepare countermeasures,"the official said.
The official said reports of potential cheating in the certified public accountant exam were concerning. The presidential chief of staff for policy was to convene an emergency meeting later on Sunday afternoon.
Prime Minister Han Seong-sook acknowledged on Friday public concern over AI-assisted cheating. A statement from her office said the government would"prepare thoroughly in advance and closely oversee the process until the very end to ensure the college entrance exam is conducted fairly and stably".
Officials said they would keep watch through the end of the college entrance exam so it could be held fairly and without disruption. Police have the tax-exam confession, but have not announced any further outcome.