Kim Song, North Korean representative to the UN, wrote the letter to Guterres seeking "urgent" mediation.
Pyongyang described the seizure of the freighter Wise Honest as "unlawful, outrageous and sovereignty-infringing" act, which indicated that the US was "indeed a gangster country", according to the state-controlled Korean Central News Agency.
The US Justice Department said on May 9 it captured the 17,061-tonne vessel, suspected of transferring coal and machinery in violation of sanctions, in the first such direct seizure by American authorities.
"Recently, the US committed an unlawful and outrageous act of taking the DPRK's cargo ship to Samoa, linking the ship to the violation of the American domestic law. This act of dispossession has indicated that the US is indeed a gangster country that does not care about international laws," Kim Song stated in the letter.
Moreover, the US committed a sovereignty-infringing act of flagrantly violating the UN Charter by dispossessing the cargo ship where the DPRK's sovereignty was fully exercised, the envoy added.
The seizure added to uncertainties over resumption of stalled nuclear talks between Washington and Pyongyang.
The negotiations between the two countries hit an impasse since the second summit between US President Donald Trump and North Korean leader Kim Jong-un broke down in Hanoi in February due to failure to bridge gaps over the scope of Pyongyang's denuclearisation and Washington's sanctions relief.
North Korea also carried out missile launches recently after 17 months without conducting any weapons tests.
Trump, however, played down the importance of launch of the short-range projectiles by Pyongyang, saying he did not believe the Asian nation's leader would jeopardise a potential nuclear disarmament deal.