Sources familiar with the sale tell TMZ ... the "Pose" star listed his 4-bedroom, 4.5-bath house -- which is located 70 miles outside NYC in the beach town of Bellport Village in Long Island -- on Tuesday ... asking for a cool $2.5 million for the over one-acre property.

While it may sound like a pretty penny to some ... it's worth every cent. The house comes with a gorgeous outside patio, a heated lap pool, a home gym, and its very own tree house. The property also offers over 4,500 square feet of living space on well-kept grounds.

Now if you're wondering why Billy would part ways with such an ideal investment ... he actually shared back in August he was hurting financially, as a result of the SAG strike, and telegraphed this sort of move might happen down the line.

At the time, he said both a movie and a TV show he was working on got scrapped due to the protests ... which lasted a whopping 118 days.

There's also this ... BP and his ex-husband Adam Smith announced their split last year -- and they wouldn't be the first celebrity couple to sell their home amid a division of assets.

It looks like Billy will have a fresh start when the ink dries on his house sale.