"In Rio I fought for gold," Malachowski said on his Facebook profile. "Today I call on all people - let us fight together for something that is even more valuable. For the health of this fantastic boy."
Reigning world champion Malachowski said all the money raised from the auction would be spent on treating 3-year old Polish boy Olek Szymanski, who has suffered for nearly two years from retinoblastoma, an eye cancer affecting young children.
"There is no chance of saving Olek's eyes in Poland. The only chance is therapy in New York," Malachowski said.
A Polish foundation called Siepomaga has already gathered about a third of the roughly 480,000 zlotys (96,448 pounds) it said was needed to finance the boy's treatment at ophthalmic oncologist David Abramson's clinic in New York.
The highest bid in the auction for Malachowski's silver medal on Poland's Allegro website had reached about $6000 by 1710 BST on Friday. The auction ends on Aug. 26.
"I invite everybody to join the bidding. If you help me, my silver medal may be more valuable for Olek than gold," Malachowski said.
