is once again looking to unload his mansion in the Hamptons ... and this time, he's slashed the price over $10 million and made a video pitch for potential buyers.

The actor just relisted his massive estate in Amagansett, NY, for $18,995,000 ... which is a heavy reduction from the original $29 million price tag.

Alec's longtime Hamptons home is 10,000 square feet with 7 bedrooms ... the modern farmhouse is on a 10-acre lot and he says, "You just can't get this out here anymore, you can't buy big pieces of land, especially in Amagansett.

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The place is decked out with a movie theater, wine tasting room and wood-paneled library, plus an outdoor pool and spa. It's also got several balconies and screened-in porches to enjoy the surrounding nature preserve.

In his pitch video, Alec waxes poetically about his love for the Hamptons ... he says he fell in love the moment he first went there, and says he used to go out to the nearby beach a lot when he was younger, sometimes falling asleep under the stars.

Alec doesn't actually go inside the home in his video, he's just on the property talking about how much he enjoys the Hamptons .

.. and he even shows some of his old homes in the area.

The house first went on the market in September 2022 but it never sold and a year later it was taken off the market. Now, AB and co. are trying to get it going again -- and you can tell ... they really wanna unload this thing, evidenced in Alec's involvement and the price slash.

Alec's moved to greener pastures -- heading north to his 55-acre farm in Vermont ... but he makes it sound like he'll really miss this place. Scott Bradley of Saunders & Associates has the listing.