Boris Grigoryevich Onishchenko (Russian and Ukrainian: Борис Григорьевич Онищенко, Ukrainian pronunciation: [boˈrɪz ɦrɪˈɦɔrʲewɪtʃ onɪʃˈtʃɛnko]; also transliterated as Onyshchenko, Onishenko, Onischenko; born 19 September 1937) is a former Soviet modern pentathlete who competed at the 1968, 1972 and 1976 Summer Olympics.

From 1967 to 1974, he competed in six World Modern Pentathlon Championships, winning gold five times as an individual or as part of the Soviet squad.

He is best known for his disqualification from the 1976 Summer Olympics and subsequent lifetime ban for cheating by means of an illegally modified épée.

Although he was rated fourth, which would relegate him to being a reserve, Onishchenko won a preliminary tournament in the Soviet Union that pushed him onto the team.

It was found that Onischenko's épée had been illegally modified to include a switch that allowed him to close this circuit without actually depressing the tip of his weapon, so he could register a touch without making any contact on his opponent.

[2] After his lifetime ban from the sport that was issued by the International Olympic Committee, Onishchenko worked as a taxi driver in Kyiv and competed in Soviet national events.