Toulon centre Nonu, who played more than 100 times for the All Blacks, was shown a yellow card in Saturday's narrow 28-27 last 16 victory over South Africans Stormers for a high tackle on Wandisile Simelane.
He was cited after the match.
"Nonu accepted that he had committed an act of foul play that warranted a red card," the EPCR said in a statement.
"The (disciplinary) committee upheld the citing complaint, finding that Nonu had tackled Simelane in a dangerous manner and determined that the offending was at the mid-range of World Rugby's sanctions and six weeks was selected as the appropriate entry point."
The 43-year-old's suspension was reduced by half due to "his expression of remorse and his very good disciplinary record", the statement said.
He will miss Toulon's quarter-final trip to Glasgow Warriors this weekend but will be available to play again for Toulon on April 27.
Their next match after that will be a home Top 14 clash against French champions Toulouse.