For more than a year, Brahimi has made no secret that he is contemplating stepping down from the post as the United Nations and Arab League joint special representative on Syria. Brahimi is due to brief the United Nations Security Council later on Tuesday.
"It's not very pleasant for me. It's very sad that I leave this position and leave Syria behind in such a bad state," Brahimi told reporters after Ban made the announcement.
Ban said he would work on finding someone to replace Brahimi to try to bring an end to the three-year civil war in Syria.
"At this time, I have to think who should be the right person and at what time," he told reporters.
There are several possible candidates to replace the veteran Algerian diplomat, diplomatic sources have said, among them former Tunisian Foreign Minister Kamel Morjane.
Brahimi has organized two rounds of negotiations in Geneva between Syrian President Bashar al-Assad's government and members of the opposition seeking to oust him.
While there were no breakthroughs at those talks, diplomats and U.N. officials said that Brahimi had wanted to continue the Geneva process to find a negotiated solution that would end the fighting, launch a political transition and begin the process of reconciliation between the supporters and opponents of Assad.
But Syria's April 21 announcement that it will hold presidential elections on June 3 dealt a severe blow to Brahimi's efforts in Geneva, diplomats said, since the vote is widely seen as a bid by Assad to defy widespread opposition and extend his grip on power.
Brahimi told reporters a year ago in New York that he thought about resigning every day.