Luxembourg anti-doping authorities backdated the suspension, sidelining Schleck until July 14 when the Tour enters its last week.
Schleck, who finished third on the 2011 Tour, walked out of the race last year after testing positive for the diuretic Xipamide.
Schleck, 32, is one of the top riders in the peloton and a proven winner both in one-day classic and multistage events.
The RadioShack Nissan Trek leader stepped out of the Tour last July 17, five stages from the end and three days after testing positive.
"The decision to suspend me one year is too severe considering the fact that the council acknowledged that I unintentionally consumed a contaminated product," he said in a statement.
Schleck was facing up to a two-year suspension but the Luxembourg anti-doping authorities shied away from that.
"The fact that he never was condemned and never tested positive during the countless doping controls he underwent, allows the disciplinary council to consider that a 12-month suspension is proportional," the ruling said.
Xipamide is classified as a specified substance, which the World Anti-Doping Agency defines as those that are "more susceptible to a credible, non-doping explanation." Bans for such substances are often shorter, and athletes have a better chance of proving that they did not intend to consume it or enhance their performance.
Schleck's team said it would study the verdict before announcing further steps. It could appeal to the Lausanne-based Court of Arbitration for Sport. WADA and the International Cycling Union can also appeal to seek a tougher sentence.
The suspension is another blow to cycling, coming on the heels of the admission of Lance Armstrong this month that he doped his way to seven Tour de France victories.
Frank and his brother Andy have been Luxembourg's most popular cyclists over the past decade. Andy Scleck was awarded the 2010 Tour victory after Alberto Contador was stripped of the title because of a doping violation.
Frank won alpine stages in the 2006 and 2009 Tours, and he also won the Amstel Gold Race one-day classic in 2006.