The Seoul Central District Court found Lee guilty of receiving 30 million won ($27,400) from a businessman in 2013.
The eight-month prison term was suspended for two years.
Lee stepped down in April last year after the businessman, Sung Wan-Jong, revealed in a newspaper interview that he had given the 30 million won to the future prime minister while he was still just an MP.
Sung later committed suicide, leaving a note in which he listed the names of other officials, including current and former senior aides to President Park Geun-Hye — suggesting he had also given them money.
Bribery scandals involving politicians and rich businessmen have been a fixture in South Korean politics for decades.
Two former presidents served prison terms for taking bribes and dozens of heads of major business groups have been convicted of forming slush funds to lobby politicians.