The Williams driver told Reuters before qualifying for Sunday's Belgian Grand Prix that he had been voted in by other drivers to join compatriot Felipe Massa and Germany's Sebastian Vettel.
Heidfeld, who was the Mercedes GP team's reserve driver until vacating that position to take up a new role as tyre tester for future supplier Pirelli, had also been chairman of the GPDA.
Barrichello, Formula One's most experienced driver, is celebrating a record 300th grand prix in Belgium this weekend.
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Horner says he is committed to Red Bull as Ferrari seek new team principal
The GPDA acts as the drivers' body in talks with the governing FIA and F1's commercial supremo Bernie Ecclestone.
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