If built, the Nord Stream 2 pipeline would supply around 55 billion cubic metres of natural gas a year from Russia to Germany under the Baltic Sea, circumventing Poland, the Baltic states and Ukraine which all oppose the project.
Poroshenko was cited as saying that the gas pipeline is "a political bribe for loyalty to Russia" while it is designed to impose an economic and energy blockade on Ukraine and seriously harm that country.
The pipeline does not have any economic justification, Poroshenko was quoted as saying.
He added that a transit gas pipeline running through Ukrainian territory is "much cheaper" and could be easily and cheaply modernised.
Its current capacity of up to 100 billion cubic metres of natural gas a year could be easily increased, Poroshenko was cited as saying.
"Please ask us instead of investing 10 billion dollars in Nord Stream 2," he added.
Poroshenko was expected to make a trip to Berlin on Tuesday.
The U.S. State Department spokeswoman said last month that the U.S. government opposed Nord Stream 2 as the project would undermine Europe’s energy security and stability.
Heather Nauert said at a department press briefing that companies engaged in the construction and financing of the pipeline "could expose themselves to sanctions" under U.S. law.
Nord Stream 2 is a gas pipeline project intended to secure the transport of natural gas from Russia to Germany across the Baltic Sea, bypassing transit countries.
On March 27, 2017, Naftohaz Ukrainy filed a claim to the European Court of Justice demanding European Commission to revoke its decision of Oct. 28, 2016 which allows the Russian Gazprom to significantly increase the use of OPAL gas pipeline and boost gas deliveries through the Nord Stream.
In December 2016, Ukrainian President Petro Poroshenko and Polish President Andrzej Duda made joint statement calling to review the European Commission's decision, which provided Gazprom the opportunity to increase gas supplies to Europe bypassing Ukraine.
Read more: EU's new sanctions against Russia may freeze Nord Stream II project, - MEP Weber
The OPAL gas pipeline connects the offshore Nord Stream pipeline with the gas infrastructure of the Czech Republic. It started operating in 2011.