As the pair went head-to-head for the customary pre-fight photo in a Munich sports shop, outspoken Londoner Chisora - wearing a Union Jack bandana - showed the inside of his palm to the defending WBC champion.
The 40-year-old Klitschko, whose younger brother Wladimir beat another London boxer David Haye on a unanimous points decision last year to defend his four heavyweight titles, did not react to the slap.
The WBC boxing board fined Chisora $50,000 for the incident, with president Jose Sulaiman branding it an "absolute disgrace."
Chisora, 28, has a reputation for pre-fight histrionics, having once planted a kiss on the lips of an opponent in a similar situation.
Most present were quick to criticise Chisora’s actions - even his own people.
"We're all scratching our heads about it, to be honest," promoter Francis Warren, son of Frank Warren, said. "You always expect something from Dereck - he's a character - but it's usually something that winds people up or is a bit jovial.
"So for him to get physical and aggressive, unprovoked, is surprising. But I'm sure there's method to his madness and we'll find out if the madness has paid off tomorrow night."
Chisora’s trainer Don Charles criticised his charge’s actions.
"I'm not Dereck Chisora. Like I said, it's unacceptable," he said to reporters afterwards.
The incident overshadowed the weigh-in, which saw Chisora tip the scales at 17st 3lbs, three pounds lighter than Klitschko.