"The decision to exclude the athlete from the 2012 Olympic Games shall be set aside and the Spanish athletics federation and the Spanish Olympic Committee shall confirm his selection in the Spanish Olympic team," the ad hoc division of the Court of Arbitration for Sport said in a statement on its website.
It said the full decision would be published on Wednesday.
Mullera was dropped from the Spanish team earlier in July after a newspaper published an email exchange between an address in his name and an unidentified doctor in which a possible doping plan was discussed.
The Spanish athletics federation (RFEA) opened disciplinary proceedings against Mullera, national steeplechase champion in 2010, over a possible doping offence.
- Spanish Olympic Committee