Schleck, third overall in the 2011 Tour, tested positive for the diuretic Xipamide following the 13th stage.
ALAD representative Raymond Mousty told a news conference that the ban started from the day Schleck failed the test on July 14, 2012.
Schleck, who has denied knowingly taking a banned substance, will be eligible to compete again two weeks after the start of the Tour.
"Of course I am disappointed by the verdict that has just been announced. I think that the decision to suspend me during one year is too severe considering the fact that the Council acknowledged that I unintentionally consumed a contaminated product," Schleck said in a statement.
"Unfortunately the provisions of the UCI are such that an involuntary contamination is sufficient in order to pronounce a punishment.
"We will now analyse the decision in detail and decide on potential further steps. However I bear a positive aspect of the decision in mind, the judges acknowledged that I am not a cheater."
The 32-year-old rider, who also finished fifth overall in the 2008 and 2009 Tours, faced a possible suspension up to two years.
In its reasoned decision, ALAD explained that it opted for a one-year ban on grounds of proportional punishment, stating that it was Schleck's first doping offence.
The disciplinary committee stated that "the presence of Xipamide is explained by the ingestion of (nutrition) supplements" and that the substance had not been "used as a performance enhancing substance or a masking agent".
Schleck can appeal to the Court of Arbitration for Sport (CAS). The International Cycling Union (UCI) and the World Anti-Doping Agency (WADA) can also refer the case to CAS if they want a longer ban.
In a similar case, the Spanish federation cleared Alberto Contador after he had failed a dope test on the 2010 Tour de France which he won, only for CAS to hand him a two-year retroactive ban after the UCI and WADA appealead.
According to the WADA code, "the Athlete has the possibility to avoid or reduce sanctions if the Athlete can demonstrate that he or she was not at fault or significant fault."
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