In the dying embers of the Stormers' heartwrenching 28-27 loss to Toulon in the Champions Cup Round of 16 on Saturday, Nonu clattered into centre Wandisile Simeline, forcing a head knock.
The double World Cup winner was only shown a yellow card for the foul play, but following a citing for the incident, European rugby's governing body has acted.
"Nonu accepted that he had committed an act of foul play that warranted a red card," the EPCR said in a statement.
"The 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 added.
Nonu is free to play again on 27 April. However, if he applies for and completes the World Rugby Coaching intervention programme, he will be free to play on 20 April, meaning he could feature in the Champions Cup semifinals should Toulon defeat the Glasgow Warriors in Scotland this weekend.