A month after vacationing with Vermont Sen. Bernie Sanders, Mayor Bill de Blasio held a sit-down at City Hall with another national progressive leader — Massachusetts Sen. Elizabeth Warren.

The one-hour meeting wasn’t on the mayor’s public schedule, and City Hall officials declined to detail what the duo discussed, calling it a private meeting.

Warren entered and exited City Hall through a rarely used side entrance, which allowed her to avoid members of the press on her way out.

Warren is among the top Democrats being discussed as a possible candidate to take on President Trump in 2020.

The get-together comes just weeks after the mayor launched a federal political action committee to help influence this year’s state and national elections, and to pick up travel expenses for him and his wife, Chirlane McCray.

Hizzoner was also scheduled to meet on Tuesday with former US ambassador to South Africa, Patrick Gaspard, a longtime pal and adviser.

Gaspard, president of George Soros’ Open Society Foundations, is one of the five outside advisers that City Hall dubbed “Agents of the City” in order to shield their communications with the mayor from the public.

The Post and NY1 won a lawsuit earlier this year that forced the release of those emails — many of which are still pending.