The country?s top four lenders ? Piraeus Bank, National Bank, Alpha Bank, and Eurobank ? were all down by about 30%.
The bourse was shut just before Athens imposed capital controls at the height of the debt crisis.
Traders had predicted sharp losses as a result of pent-up trading.
Banks account for about a fifth of the main Athens index.
Although Greece struck a bailout deal with its creditors last month, political in-fighting in Athens over the conditions could still result in prime minister Alexis Tsipras calling an early election.
The Greek economy has begun to reverse the gains it was making before Mr Tsipras?s Syriza-led coalition took power in January on an anti-austerity platform.
The European Commission expects Greece to go back into recession this year, with the economy contracting by between 2% and 4%.
The Greek economy was in recession for six years until 2014.