But the charge was reduced to a misdemeanour, and he received a one-year suspended prison sentence, escaping a longer term.
He was acquitted of a second charge of breach of trust.
The "Lagarde list" of HSBC bank account holders caused outrage in Greece as it faced its economic crisis.
Greek governments have been accused of not fully investigating the list, which was given to Athens in 2010 by the then French Finance Minister Christine Lagarde.
It is an extract of a list of account holders at a Geneva branch of HSBC leaked by former bank employee Herve Falciani.
But the Pasok government took no action after receiving it and later claimed to have lost it.
Papaconstantinou, who was finance minister at the time, denied any wrongdoing.
The court in Athens found him guilty of doctoring a document, but the charge was reduced from a felony to a misdemeanour.
Papaconstantinou was expelled from Pasok in 2012 after the allegations emerged.
The country's new leftist-led government of Prime Minister Alexis Tsipras has promised to crack down on tax evasion.
Former Greek Finance Minister George Papaconstantinou has been found guilty of removing relatives' names from a list of potential Greek tax evaders.