Radio Caroline is a British radio station founded in 1964 by Ronan O'Rahilly and Allan Crawford, initially to circumvent the record companies' control of popular music broadcasting in the United Kingdom and the BBC's radio broadcasting monopoly. Unlicensed by any government for most of its early life, it was a pirate radio station that never became illegal as such due to operating outside any national jurisdiction, although after the Marine, &c., Broadcasting (Offences) Act 1967 it became illegal for a British subject to associate with it.

The Radio Caroline name was used to broadcast from international waters, using five different ships with three different owners, from 1964 to 1990, and via satellite from 1998 to 2013. Since August 2000, Radio Caroline has also broadcast 24 hours a day via the internet and by the occasional restricted service licence. Currently, the station broadcasts on 648 AM across much of England and DAB radio in certain areas of the UK: these services are part of the Ofcom small-scale DAB+ trials. Caroline can be heard on DAB+ in Aldershot, Birmingham, Cambridge, Brighton, Glasgow, Norwich, London, Portsmouth, Poulton-le-Fylde and Woking on digital radio. Caroline can also be listened to over the internet.

In May 2017, Ofcom awarded the station an AM band community licence to broadcast on 648kHz to Suffolk and north Essex; full-time broadcasting, via a previously redundant BBC World Service frequency and transmitter mast at Orford Ness, commenced on 22 December 2017.^{[citation needed]}

Radio Caroline broadcasts music from the 1960s to contemporary, with an emphasis on album-oriented rock (AOR) and "new" music from "carefully selected albums". On 1 January 2016, a second channel was launched called Caroline Flashback, playing pop music from the early 1950s to the early 1980s.

Radio Caroline was the brainchild of the Irish musician manager and businessman Ronan O'Rahilly, the idea being formulated following O'Rahilly's failure to obtain airplay for the records of one of his contracted artistes, Georgie Fame, on Radio Luxembourg or the BBC Light Programme. At this time it was Radio Luxembourg policy to only promote sponsored programmes funded by major record labels: EMI, Decca, Pye and Philips.

Undeterred by this failure, and encouraged by Scandinavian and Dutch radio pirates, in February 1964 O'Rahilly obtained the former Danish passenger ferry Fredericia which was subsequently taken to the Irish port of Greenore, which was under the ownership of O'Rahilly's father, Aodogán, in order for the vessel to be fitted out as a radio ship.

This was a busy time at Greenore with the work to the Fredericia being carried out in tandem with Allan Crawford's "Project Atlanta", which saw a similar conversion undertaken on the Mi Amigo.

Financial backing for the venture came from six investors, including John Sheffield (chairman of Norcros); Carl "Johnny" Ross (managing director of Ross Foods) and Jocelyn Stevens of Queen magazine, with which Radio Caroline shared its first office.