"I have returned to rescue the country," he said.
Sam Rainsy was cheered at the airport and along a route to a park where he spoke to supporters,Time magazine reported.
The opposition leader arrived with the country's general election campaigning in full swing in advance of July 28 elections.
Sam Rainsy was convicted in absentia for racial incitement and destruction of property -- charges he maintains were politically motivated -- and spent the last four years living in exile until a royal pardon led to his return as head of the Cambodia National Rescue Party.
Longtime Prime Minister Hun Sen has faced pressure from the international community to allow a free and fair election, but allowing his rival's return has puzzled observers, Time said.
Time reported the CPP controls every form of governance, from the judiciary to law enforcement.
Cambodia is afflicted by "systemic corruption so we have to put our house in order," Sam Rainsy told the news magazine before his return.
CNRP officials told Time people lined the street from the airport to Freedom Park, where Sam Rainsy addressed the crowd, and the 10-mile trip took 5 hours.
"I think we are entering a new phrase in Cambodia history," Sam Rainsy told Time by phone. "It is the beginning of something like the Arab Spring."