Transactions were made using the virtual currency Bitcoin, and Mr. Ulbricht, operating under the pseudonym Dread Pirate Roberts, took in millions of dollars in commissions, prosecutors have said. They said Mr. Ulbricht had “developed a blueprint for a new way to use the Internet to undermine the law and facilitate criminal transactions,” and that his conviction was “the first of its kind, and his sentencing is being closely watched.”

Judge Forrest echoed that message. “What you did was unprecedented,” she told Mr. Ulbricht, “and in breaking that ground as the first person,” he had to pay the consequences. Anyone who might consider doing something similar, the judge added, needed to understand clearly “and without equivocation that if you break the law this way, there will be very serious consequences.”

Mr. Ulbricht was convicted in February on charges that included engaging in a continuing criminal enterprise and distributing narcotics on the Internet, each of which carried potential life terms. The government had also alleged that he had solicited the murders of people he saw as threats to his operation, and that at least six deaths were attributable to drugs bought on the site.

“Make no mistake, Ulbricht was a drug dealer and criminal profiteer who exploited people’s addictions and contributed to the deaths of at least six young people,” Preet Bharara, the United States attorney for the Southern District of New York, said in a statement.

At the hearing, the mother of one such victim and the father of another each delivered emotional statements to the judge. The father, identified as Richard, whose 25-year-old son had died after using heroin that prosecutors said was purchased on Silk Road, accused Mr. Ulbricht of being driven by greed in making drugs easily available to vulnerable people.