Errani, who has won nine singles titles, reached the singles final of the 2012 French Open and won five doubles grand slam titles, will be able to return to competition on October 3.
Letrozole can help increase body mass and has been banned for all athletes since 2005 following concerns from the World Anti-Doping Agency it was being abused by bodybuilders.
The former world No 5 has been suspended for two months for the anti-doping violation, which she claims came from eating pasta accidently contaminated by her mother's breast cancer medication. However, the tribunal did say there lacked evidence this drug could be performance-enhancing for an elite tennis player.
The former world number five will be out of action for the next two months meaning she will be out of the US Open at Flushing Meadows later this month.
In a statement released on her official Twitter account, Errani strenuously denied knowingly taking a banned substance.
Additionally, her results between February 16 (date of sample collection) and June 7 (date of her next test, which was negative) should be disqualified, the Independent Tribunal ruling on the case determined. At a hearing in July, Errani, alongside her mother, testified that she did not knowingly ingest the drug, and that accidental contamination from her mother's medicine, which she used to treat breast cancer, was to blame.
Errani reached the singles final at the French Open in 2012, losing to Russian Maria Sharapova 6-3, 6-2.
The tennis player said she never took any letrozole and suspected that "accidental food contamination occurred at some stage in the house". "The only hypothesis is food contamination". "I am extremely disappointed but at the same time at peace with my conscience and aware I haven't done anything wrong".
Some will question why she was allowed to play for nearly four months in between those dates without undergoing another test, despite a busy period in which she played in 10 tournaments and enjoyed a successful run through qualifying at the French Open.
The written reasons for the verdict revealed Errani had been visiting her mother prior to the date of her positive test on February 16.