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.

The Faraday Lecture Block dates back to the 1960s and the university says its "tired and monotonous design" means it has become a bit of an eyesore.
As well as new multi-coloured external glass cladding, the building will have a new entrance hall, extended second floor and improved access.
Work on the 11-month project is due to start in January.
The university said the new look would add vibrancy to the building and the mall area.
The majority of construction will take place during the Easter holidays and summer months.
A spokeswoman said: "It'll create a good impression and reflect the standards that Swansea holds and a professional atmosphere that fits in with the degrees that are taught in that building".
Former Greek Finance Minister George Papaconstantinou has been found guilty of removing relatives' names from a list of potential Greek tax evaders.
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One of the main buildings on Swansea University's Singleton Park campus is to undergo a £2.75m makeover.