Brian Farrell admitted conspiring to distribute heroin, cocaine and methamphetamine, in March this year.
Silk Road 2.0 was shut down in 2015, following Operation Onymous, raids that targeted almost 400 suspected illicit marketplaces around the world.
At the time, it was generating sales of about $8m (Â£5.5m) a month and had about 150,000 users.
Ross Ulbricht, who set up the original Silk Road, was sentenced in May 2015 to life in prison on several charges including distributing drugs.
Silk Road 2.0 was a copy of the original site and allowed people to use bitcoins to anonymously buy and sell drugs and other illegal goods and services via the Tor dark web network.
Farrell, who operated online using the alias "DoctorClu", had admitted his involvement with the site when prosecutors had been searching his Washington home in 2015, court papers revealed.
Blake Benthall, arrested in 2014, denies creating the Silk Road 2.0 site.
An American who helped run a successor to the Silk Road drug marketplace has been sentenced to eight years in jail.