Leno, a guild member, was not among the three writers disciplined.
The former "Tonight Show" host was accused of writing his own nightly monologues while writers were on the picket lines.
"All disciplinary matters related to the strike have now been resolved by the board," the guild said in the letter. Officials declined further comment.
Leno's publicist, Dick Guttman, said the decision confirmed the host's position all along that he had broken no rules.
"He felt that the rules justified it," Guttman said. "It turns out they did."
According to the contract, "material written by the person who delivers it on the air" is exempted from the agreement. The exception applies to shows outside prime-time, which includes NBC's "Tonight Show."
Leno will debut his 10 p.m. nightly variety show, "The Jay Leno Show," on NBC on Sept. 14.
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