The justice department said the ship was used to transport coal, North Korea's largest export but subject to a UN export ban.
The vessel was initially impounded in Indonesia in April 2018.
It is the first time the US has seized a North Korean ship for breaching sanctions and comes amid worsening relations between the two.
A meeting between North Korean leader Kim Jong-un and US President Donald Trump ended without agreement in February with the US insisting North Korea give up its nuclear programme and Pyongyang demanding sanctions relief.
