The suspension will cost Johnson about $95,000 of his $650,000 salary.
"The league has made clear that deliberate blows to the head -- particularly those which cause injury to the head of an opponent -- will be treated severely," said NHL senior vice president Colin Campbell, who issued the suspension following a hearing Monday.
Johnson got a match penalty for punching Beukeboom in the back of the head during a game last Thursday. Beukeboom collapsed to the ice face-first and was still wobbly when he left the ice with help from medical personnel a few minutes later.
Johnson has already served one game of the suspension and will miss the Kings' next 11 games.
The suspension is the longest in the NHL since Tony Granato of the Kings was suspended 15 games for slashing Pittsburgh's Neil Wilkinson Feb. 9, 1994.
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