The England international hooker was reported for striking his opposite number Rory Best during last Friday's Pool Four encounter at Franklin's Gardens.
Hartley will miss Saturday's Ulster return fixture in Belfast, plus Northampton's Aviva Premiership appointment at home to Harlequins on December 22.
In a statement, European Rugby Cup said Hartley pleaded guilty to the offence.
Welshman Roger Morris, an independent judicial officer, heard evidence and submissions by video conference from Hartley and his legal representative, in addition to ERC disciplinary officer Roger O'Connor.
ERC said: "The judicial officer upheld the citing and employing the IRB (International Rugby Board) recommended sanctions for Law 10.4(a), determined that the offence was at the low-end entry point of two weeks.
"When considering relevant aggravating factors, the judicial officer added one week for the player's disciplinary record and then subsequently reduced the suspension by one week for the player's conduct during the hearing and guilty plea.
"Hartley will be free to play on 24 December 2012. Both the player and ERC have the right to appeal the decision."
Hartley, 26, recently returned to action after missing England's entire autumn series of Tests through injury. Leicester's Tom Youngs impressed as the starting hooker in his absence.
New Zealand-born Hartley has a chequered disciplinary history.
He received a 26-week ban in 2007 after being found guilty of gouging two Wasps players in a Premiership match, and earlier this year he was cited for biting Ireland flanker Stephen Ferris during a Six Nations game that saw him land an eight-week suspension.
Northampton, now without Hartley, must beat Ulster this weekend to keep alive their fading Heineken Cup quarter-final hopes.
Meanwhile, Wales international scrum-half Lloyd Williams was banned for five weeks when he appeared at a disciplinary hearing in Dublin.
The Cardiff Blues player was sent off for a tip tackle on his opposite number, Montpellier's Benoit Paillaugue, during a Heineken Cup clash last Sunday.
Williams will be sidelined until January 14.
Judicial officer Jeremy Summers ruled that the offence warranted a mid-range entry point of an eight-week ban.
He added one week as a deterrent, but then took a maximum of four weeks off due to mitigating factors - a guilty plea, the player's disciplinary record and what ERC described as his "clear remorse".
Newport Gwent Dragons flanker Jevon Groves was suspended for two weeks until December 24.
He was sent off in the Dragons' Amlin Challenge Cup fixture against Italian side Mogliano last weekend for a tip tackle, but Summers amended the offence to that of a dangerous tackle.
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