Sam Rainsy, who has led the Cambodia National Rescue Party (CNRP) since its inception but fled to France in 2015 in the face of several lawsuits against him, announced his resignation in posts on Twitter and Facebook.
The sudden move throws doubt over a party that poses the only major challenge to strongman Hun Sen's grip on power ahead of 2018 elections.
"I resign as CNRP leader for the sake of the party. In all circumstances I cherish and uphold the CNRP’s ideals in my heart," he wrote.
Rainsy's resignation announcement comes shortly after Hun Sen, who has loomed over Cambodian politics for 32 years, proposed amending a political party law to bar convicts from serving as party leaders -- a clear threat to Rainsy and the CNRP.
In December Rainsy was sentenced to five years in prison over a post on his Facebook page -- a conviction that made any imminent return from exile unlikely.
Rights groups say the onslaught of legal cases, which have also netted other opposition figures, are politically-motivated and part of Hun Sen's broader crackdown on dissent ahead of elections he is bent on winning.
The party's spokesman Yim Sovann told AFP he had no other information about Rainsy's decision to step down, saying only that it was motivated by "personal reasons".
The CNRP opposition party made huge gains in the 2013 elections and say they only lost because the vote was stolen -- a claim Hun Sen has vigorously rejected.
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