Best admitted illegally making contact with the eye or eye area of Wasps and England back-row star James Haskell with his hand during a Guinness Premiership game on September 20.
Best will miss eight rounds of Premiership action, plus all of Northampton's European Challenge Cup and EDF Energy Cup pool matches.
Best, capped 18 times by Ireland and a member of their 2007 World Cup squad, appeared before a three-man panel chaired by the RFU's disciplinary officer His Honour Judge Jeff Blackett.
Best had been cited by Premiership citing officer Ken Pattinson following Saints' 24-20 victory over Wasps at Franklin's Gardens.
Haskell gave evidence before the panel, confirming his written statement made after the game that the player (Best) "gratuitously commenced to gouge at (his) eye with increasing intensity".
Haskell, a contender to captain England in their autumn Tests next month, said his vision became increasingly blurred, he suffered excruciating pain and felt physically sick because of soreness.
He was later taken to Northampton General Hospital, where an ophthalmologist diagnosed a corneal abrasion and prescribed antibiotic drops.
Photographs taken a day after the game showed significant swelling around Haskell's eye, which was closed and bruised, with a one-centimetre scratch on his upper eyelid.
Best, whose Northampton team-mate Dylan Hartley received a six-month suspension for eye-gouging last year, told the panel he had acted recklessly, and not deliberately.
The panel said it accepted Best had not intentionally searched for Haskell's eye and did not intend to inflict serious injury, but he had tried to grab somewhere on his head to pull him up and backwards.
But they did not agree with Best's account that contact with the eye was "fleeting".
The 29-year-old joined Northampton from Ulster this summer. During a professional career of more than 150 games he had never previously been cited.
Best and his club have until midday tomorrow to lodge any appeal with the RFU.
After the RFU announced their punishment, Northampton issued a statement that read: "In the light of the detail of the judgement, the club and player are in the process of seeking legal advice and will make a further statement during the course of today."