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.
North Korea has carried out two weapons test in the space of the past week in what is widely seen as an attempt to increase pressure on the US over its failure to make concessions.