Gwyneth Paltrow Was Upset by Brad Pitt's Marriage to Jennifer Aniston, New Book Claims

Gwyneth Paltrow wasn't wishing Brad Pitt her best when he tied the knot with Jennifer Aniston ... at least, according to a new biography on the A-list actor.

Amy Odell, a biographer who wrote "Anna: The Biography" in 2022, has a new book out July 29, all about the actress and Goop founder, based on interviews she says she conducted with people in Paltrow's orbit. As far as we know, she didn't speak with GP for the book.

In the book, Odell says Paltrow told friends she was sad ... adding that she regularly blasted his taste in women around this time as well.

She allegedly threw more shade at Pitt during a business meeting in 2000 with Estee Lauder cosmetics heiress Aerin Lauder ... with Odell claiming, "At one dinner, the two were talking about Brad Pitt. According to someone familiar with the discussion, Gwyneth allegedly told her, ‘He’s dumber than a sack of s***.'"

We've reached out to Paltrow about these alleged comments ... so far, no word back.

While Odell says Paltrow was firing off about Pitt in her private life, she notes Gwyneth was more cagey publicly ... shutting down an interview question about Brad at the Toronto Film Festival in September of 2000.

Paltrow and Pitt dated from 1994 -- after meeting on the "Se7en" film set -- until 1997. They got engaged in '96 before splitting up. Pitt and Aniston started dating in 1998 and were married from 2000 to 2005. Paltrow's opened up about her sex life with Brad in recent years ... which she says was full of passion.

This book has a lot more juicy claims in it, BTW ... from an inside look at Gwyneth Paltrow and Ben Affleck's alleged sex life to the "consciously uncoupling" she went through with Chris Martin.