LOS ANGELES — In a gift to many musicians, moviemakers and other content-creators, a federal judge has found that the song “Happy Birthday To You” is entirely in the public domain.

It could mean millions lost for the music publishing company that has been collecting on the copyright to one of the most widely sung songs in the world.

U.S. District Judge George H. King found Tuesday that the song’s original copyright, obtained by the Clayton F. Summy Co. from the song’s writers and bought for $15 million in 1988 by Warner/Chappell Music, only covered specific piano arrangements of the song and not its lyrics.

The basic “Happy Birthday” tune, derived from another popular children’s song, “Good Morning to All,” has long been in the public domain, and King’s decision, though it could be appealed, makes the entirety fair game for use.

One of the co-plaintiffs, Ruypa Marya of the music group Ruypa & The April Fishes, also praised the decision as momentous, saying “I hope we can start reimagining copyright law to do what it’s supposed to do — protect the creations of people who make stuff so that we can continue to make more stuff.”