Guy Wildenstein and seven other defendants were accused of trying to hide millions of euros in assets.
The presiding judge of a Paris court said there had been a "clear attempt" at concealment.
But he acquitted them because of shortcomings in both the investigation and French tax fraud legislation.
Guy Wildenstein gave a rare interview three months ago in which he denied wrongdoing and said he hoped he would not be made into a "scapegoat".
The Wildenstein family estate includes famous paintings, Caribbean properties, racehorses, and a Kenyan ranch.
The level of secrecy around the family fortune was first uncovered in the late 1990s during messy divorce proceedings between Guy's brother Alec and his then-wife, Jocelyn.
Ten years later in 2008, after Alec's death, Guy Wildenstein declared an inheritance of $61m (Â£50m; â‚¬57m).
But repeated claims by other women who had married into the family led investigators to look again at the Wildenstein finances in 2010.
The patriarch of an international art-dealing dynasty has been cleared of concealing paintings and properties from the French tax authorities.