Khan, the leader of the Opposition, on Sunday had confirmed that he had proposed marriage to the woman named Bushra Maneka but denied reports that he had secretly wed her.
Sharif said Khan was cowardly hiding after putting the dignity of a family at stake.
"What sort of person he is who hides like a coward while leaving an innocent family to face rumours," he said.
Khan had first tied the knot with Jemima Khan on May 16, 1995 that ended in divorce after nine years on June 22, 2004. His second marriage was with Reham, then a TV anchor, which continued barely 10 months. Although that marriage was publicly announced on January 8, 2015, there were reports that a nikah had also taken place in early November 2014.