The 82-year-old conservative, who led Germany from 1982 until 1998, only makes rare public appearances and has been visibly in poor health for some time.
His office manager Marion Scheller confirmed on Monday that he had the heart surgery in February.
“Due to a cardiological problem he was doing very poorly,” she said. But she added that he was feeling much better since the surgery and was now working on two books.
Kohl played a pivotal role in the months around the fall of the Berlin Wall in November 1989 and has been known as the father of German unity. He was also a crucial figure in the progress of the European Union along with Jacques Chirac of France and Margaret Thatcher of the UK.
Kohl had championed Chancellor Angela Merkel, until she effectively pushed him from the leadership of their Christian Democratic Union (CDU) in 1999 over an unaccounted party slush fund.
His refusal to name those who had made secret donations to the party tarnished his reputation and he left parliament for the last time in 2002, largely withdrawing from public and political life.
He is expected to make an appearance at the end of September to mark the 30th anniversary of his election to chancellor, on the back of a constructive vote of no confidence on October 1, 1982.
The plan is for him to visit the CDU in parliament on September 25 – his first appearance there since leaving. Two days later he will attend a gala in his honour at the German Historical Museum where Merkel will speak.