Red sludge containing metal oxides flooded a town and two villages, killing 10 people and injuring 200.
The court cleared ex-director Zoltan Bakonyi and the other managers of alleged negligence, waste management violations and environmental damage.
They worked at the MAL plant in Ajka.
The BBC's Nick Thorpe in Budapest says there was anger in court when the verdict was read out, and an appeal is likely.
The caustic sludge spilled from a storage pond after a wall burst open. It flooded Devecser and two nearby villages, Kolontar and Somlovasarhely. The disaster affected an area of 40 sq km (15.4 sq miles).
The judge in the city of Veszprem said the MAL employees' criminal responsibility for the disaster could not be proven, and that the company itself was not on trial.
The cause of the collapse was the unstable soil beneath the storage pond, he said.
Some compensation has already been paid to the victims' families, and the Hungarian state has rehabilitated the whole valley, rebuilding many homes.
A Hungarian court has acquitted the former director of an aluminium plant and 14 other employees over a toxic spill that polluted a rural valley in western Hungary in 2010.