Bridget Moynahan is set to star in the Blue Bloods spin-off series

Donnie, 55, wrote on Instagram: "Reunited and if feels so good! So excited to have my dear friend, and TV sister @bridgetmoynahan, join me for a special guest appearance in the premiere episode of #BostonBlue! She'll also be bringing her incredible directing talents to the show this season! More fun (and surprises) to come. [eyes emojis] Let's go! [heart emojis] (sic)"

The celebrity duo previously played siblings on Blue Bloods, the hit drama series that ran from 2010 until 2024.

And Donnie previously revealed his long-term ambitions for Blue Bloods, admitting that he wanted the show to compete with Law and Order: Special Victims Unit.

Speaking to Us Weekly in 2023, Donnie said: "I mean, Ice-T and Mariska Hargitay have been going for gosh, so many years, and we hope to last as long as they have.

"One of the things I've done is I try to just stay very present in each episode and take each script that comes my way and say, 'OK, what's gonna happen now? You know, what am I doing this week?' And kind of find the journey in that particular episode and not get caught up in what's gonna happen next year, what's gonna happen in two years."

Donnie also joked about the possibility of launching a Blue Bloods spin-off.

Discussing one of his spin-off ideas, Donnie said: "Danny and Erin become private investigators and move to Hawaii, but that was too close to Magnum, P.I., so we didn't want to do that one."

Meanwhile, Bridget previously shared details of her last day on Blue Bloods, revealing that the cast were "all a little disappointed".

Speaking to People, the actress explained: "We've had such a good time. We all want to be there. It's a real loss.

"This was our daily lives for 14 years. And we've all experienced together major life moments together, whether it's births, deaths, marriages, divorces, illnesses, we've had it all together."