Lippert who started his duty as top US envoy in October 2014 was supposed to hold a "farewell" press conference at 11 a.m. in his residence located in central Seoul.
The ambassador appointed by President Barack Obama is expected to leave office soon after the Donald Trump administration takes office on Jan. 20.
An embassy official said that the press meeting had to be postponed since an urgent matter happened but did not elaborate further.
Some observers forecast that there may be no meeting with reporters before the envoy leaves the country.
The ambassador was to hold a conference with reporters on Nov. 10, a day after the US presidential election, but he canceled it on a short notice, saying that the election results had not been determined. (Yonhap)