Samantha will appear in one episode in the season finale, but will not share the screen with Carrie, Charlotte or Miranda -- the characters played by Sarah Jessica Parker, Kristin Davis or Cynthia Nixon.
Samantha is expected to speak with Carrie via phone in the cameo, which Cattrall filmed in March.
She played Samantha on the original show for six seasons from 1998 to 2004, as well as in two movies released in 2008 and 2010, but did not want to star alongside her former cast members in the sequel series amid reports of a falling-out with Parker.
Samantha's absence was explained as she had moved to London and lost touch with her besties.
An insider told The Post show staffers were "definitely shocked, very intrigued on how they're gonna write this in -- and very excited."
"She said she'd never do it! She said she'd never come back!" the source said.