The gallery's new director Gabriele Finaldi is due to take over from Nicholas Penny.
The union remains opposed to the privatisation of all the gallery's visitor services and is fighting for the reinstatement of its senior representative Candy Udwin, who an interim tribunal has found was likely to have been sacked unlawfully for trade union activity in relation to the dispute.
PSI General Secretary Rosa Pavanelli, addressed members on the first day of the indefinite strike, stating,
"We will do all we can to support your struggle. We are proud of your fight, because it is not only about defending jobs but also about defending the value of culture, that institutions like the National Gallery are a common good which cannot be sold to privateers for profit."
There will be a picket line outside the gallery in Trafalgar Square between 9am and 11am each day, and Friday from 5pm to 6.30pm.
PCS general secretary Mark Serwotka said: "We had asked the new director to step in to resolve this dispute before taking over but now his first week will be greeted by a continuous strike. We remain ready to negotiate.
"We do not believe this privatisation is in any way necessary and we fear for the reputation the gallery rightly enjoys around the world as one of our country's greatest cultural assets."
Please sign the petition: https://you.38degrees.org.uk/petitions/no-privatisation-at-national-gallery
IMPORTANT: Signatories need to enter a UK postcode so we recommend people signing from outisde the UK use the gallery's - WC2N 5DN.