"Basically, when I felt I could not help my people any more I had to move on," Danny al-Baaj, the first Syrian diplomat in Switzerland to abandon Bashar al-Assad's regime, told AFP.
"When I was involved in any negotiations (on Syria) my concern was to protect the country not the government," he added.
Baaj said he took his decision a long time ago and had been in contact with Syrian opposition group the Democratic Forum based in Paris.
He had been in Geneva for two years and met the opposition group "some time ago", before announcing his resignation last Friday, he said.
"I met the charge d'affaires (of Syria in Geneva) and I told him I had made my decision that I was going to the opposition... He said it was my choice and he wished me luck."
Speaking from Geneva where he is considering his next move, Baaj described the Democratic Forum as one of the main opposition groups. It is headed by Michel Kilo, a long-time opponent of the regime.
The development comes ahead of the release on Wednesday of an official UNHRC independent commission of inquiry report into Syria.
Baaj said he "hoped" the Geneva-based body would make progress towards consensus on the situation in Syria despite many countries letting their own agendas interfere with finding a solution.
"At the last session the HRC was very close to reaching consensus ... I hope different countries put aside their agendas to help the Syrian people," he said.
Baaj also stressed his opposition to outside military intervention in the conflict but supported the role of the UN's Supervision Mission in Syria (UNSMIS), calling it "a good thing".
"I hope it stays there. It's very important to document abuses by both sides," he said.