Felton, 34, played evil wizarding student Draco Malfoy in the Harry Potters series, which ran from 2001 to 2011, as Isaacs, 59, featured as the wicked Lucius Malfoy.
A decade after the conclusion of the Harry Potter films, it appears the actors are still close, as they were pictured on Instagram embracing as Isaacs came to support Felton in his West End debut on Saturday.
Felton stars in the play 2:22 A Ghost Story and Isaacs made sure to catch his onscreen son at the show.
As he posted a sweet photo of himself beaming with Felton, Isaacs wrote: “So proud and, along with the audience, thrilled to see my boy commanding the stage today, breaking hearts and filling pants.
“Equally terrifying, he’s recording music and has a book coming out, too. I feel smug if I take the bins out on the right day (doesn’t happen often). I’d hate him if I didn’t love him. @222aghoststory #MalfoyManner.”
Isaacs jokily added, referring to his orange complexion: “That’s a work tan btw. I haven’t caught trumphilis.”
Felton also posted a photo of himself with Isaacs and wrote: “My father heard about this play I was in @222aghoststory.”
The iconic Harry Potter duo was last seen reuniting in the HBO Max special, Harry Potter 20th Anniversary: Return to Hogwarts last year, alongside stars Daniel Radcliffe, Rupert Grint, Emma Watson and several others.
Fans were delighted to the see the pair back together, as one person wrote: “The best father and son duo ever,” while a second fan said: “Ahhh the Malfoys back together again!!”
A third chimed: “The TV father son duo that became a real-life long duo”, as another said: “This is so wholesome, I’m about to cry now.”
Ahead of his West End debut, Felton revealed that he sought advice from his previous onscreen nemesis, Radcliffe, 33, since he has starred in many West End and Broadway productions.
Felton told The One Show: “I was very excited to tell Daniel, he’s done a lot of theatre and musicals and so I was keen to get a bit of an insight.
“He advised me to embrace the madness and don’t panic and just enjoy the time actually being up there.
“It’s a very different thing than we’ve done before. I’m really excited to get started.”