It was the second arrest stemming from the release of variants of the worm, but detectives have yet to determine who created the main version.
Prosecutors refused to release any information about the suspect, not even the youth's gender or home state. They cited confidentiality rules protecting juveniles accused of crimes.
The U.S. Attorney's Office in Seattle said in a statement that the youth was arrested as a juvenile for intentionally causing damage to protected computers.
The variant the juvenile allegedly created was known as "RPCSDBOT." Prosecutors did not say how many computers they believe were infected.
Collectively, different versions of the virus-like worm, alternately called "LovSan" or "Blaster," snarled corporate networks worldwide, inundating more than 500,000 computers, according to Symantec Corp., a leading maker of antivirus software.
Earlier this month, Jeffrey Lee Parson, a high school senior from a Minneapolis suburb, pleaded innocent in Seattle to federal charges he crippled more than 7,000 computers by modifying a version of the "Blaster" worm. The charges carry up to 10 years in prison.
Federal prosecutors in Seattle are handling the cases because the worms directed infected computers to bombard Redmond-based Microsoft Corp. Web sites with e-mails.
Earlier this month, police in Romania said they were investigating Dan Dumitru Ciobanu, a 24-year-old former student suspected of modifying the worm to attack computers at a university he attended. A Bucharest computer security company said Ciobanu had been arrested, but police denied that.