The new father has become the latest celebrity to speak out on Scotland's future in the run-up to the independence referendum in September.
Cowell was reported to have said he has "Scottish blood" in him and wants the UK to "all be together".
Meanwhile, Scots actress Rose Leslie, who stars in the hit shows Downton Abbey and Game Of Thrones, said she had a "real concern about independence".
Cowell, who is to return as a judge on X Factor later this year, reportedly said: "My mum's maiden name was Dalglish so I have Scottish blood in me. Personally, I would like us all to be together.
"At the end of the day it is their decision obviously. But I would prefer us to remain one big happy family."
Leslie told The Spectator: "First and foremost, Scotland is my home and I do consider myself Scottish but I also feel very British and I hope that Scotland stays within the Union. I have a real concern about independence."
A spokesman for the pro-independence Yes Scotland campaign said: "A Yes vote is about placing Scotland's future in Scotland's hands and moving on from a Westminster system that simply doesn't have the X Factor.
"After independence, the social union between Scotland and our friends and neighbours in the rest of the UK will be stronger than ever."
A spokesman for the pro-union Better Together campaign said: "It's great to have the backing of such internationally successful people as Simon Cowell and Rose Leslie. For so many people in Scotland and right throughout the UK, it makes no sense to break up our family of nations. We have achieved so much together and we can do so much more in the future.
"Whilst the backing of well known faces isn't likely to influence how people vote, drawing attention to the debate can only be a good thing."