The MCC said the stroke had already been recognised by and dealt with by Law 36.3, and as it was a risky shot for a batsman to try and play, "it also offers bowlers a good chance of taking a wicket and therefore MCC believes that the shot is fair to both batsman and bowlers."
Pietersen said the decision was the right one for the future of the sport.
"It's important that we as players are innovative and if this shot helps make cricket more exciting and entertaining for spectators then that has to be good for the sport," he said.
"I am glad the MCC has recognised that cricket is always evolving and that this particular shot brings something special to the game.
"I have spent many hours in the nets working on it and I am pleased that all the hard work is not going to go to waste."
World governing body the International Cricket Council (ICC) had already raised concerns about the legality of the stroke before Pietersen's incredible hitting off all-rounder Scott Styris at Chester-le-Street.
The ICC's cricket committee made recommendations at its annual meeting in Dubai in May for the MCC, the custodian of cricket's laws, to review the switch hit.
And the MCC has also had reports of some club cricketers adopting a similar tactic.
Kevin Pietersen demonstrates his left-handed stance
Unlike bowlers, a batsman does not have to notify the umpires and opposing team if they opt to reverse their batting style.
However, the shot raises a number of questions for umpires, including the lbw and leg-side no-ball laws.
Despite the controversy, Pietersen believed he broke new ground with his stroke.
The first six flew over deep square leg boundary (for a left-hander) at Chester-le-Street, while the second bore more of a resemblance to a Marcus Trescothick slog sweep over the ropes at long-on.
"Reverse sweeps have been part of the game for however long, I am just fortunate that I can hit it a bit further," said Pietersen.
"Everybody wants brand new ideas, new inventions and that's a new shot. We were in a position where I needed to push the accelerator and it worked.
"On other occasions it might not work but the most important thing is a convincing England victory.
"I play to win, I would do anything to win and the most important thing is we are 1-0 up.
"The key to it is to try to raise the bar on Wednesday again, hit our straps on Saturday and if we have a good week we will have won a series."
Styris, who was on the receiving end of the shots, said he had no problem with it.
"Sometimes you've just got to take your hat off and say "well played"," he said.
"We all admire good cricket and that's what it was. I don't view it in any other light, I don't believe it's against the spirit of the game or anything, I just take my hat off and say "nice shot"."
When asked if he had been tempted to try playing the stroke himself, he said: "It crossed my mind when I was out there but I thought if I'm out, it's a double blow for me."
Former Australia captain and legendary cricket commentator Richie Benaud has also welcomed the MCC's ruling.
"Anything the MCC does I'm happy with, particularly if they let Pietersen keep on reverse sweeping sixes - that's wonderful entertainment," he told BBC Sport.
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