Volodymyr Zelensky made the comments beside Poland's President Andrzej Duda in Warsaw, where he was due to attend Sunday ceremonies marking 80 years since the outbreak of World War II.
"We hold the same position as Poland: Nord Stream 2 is unacceptable and it threatens Europe as a whole," Zelensky told reporters.
"We want the diversification of sources of energy delivery within the US, Poland and Ukraine triangle," he added.
The US, Poland and Ukraine agreed later on Saturday in Warsaw to beef up cooperation aimed at securing energy supplies in the region reliant on Russian deliveries.
The agreement is largely focused on gearing Ukraine's gas infrastructure to deliveries from the West, including US liquefied natural gas (LNG).
It was signed by visiting US Energy Secretary Rick Perry, Ukraine's Defence Minister Oleksandr Danyliuk and Piotr Naimski, the Polish government's chief energy strategist.
On Thursday, Poland's PGNiG state-run gas firm announced it was selling Ukraine's ERU energy group a shipment of LNG from the United States.
An EU and NATO member, Poland is trying to wean itself off Russian gas, notably by building an LNG terminal on its Baltic coast. It is capable of receiving LNG shipments from the US and the Middle East.
Naimski told reporters that by 2021, Poland would be able to supply Ukraine with six billion cubic metres of natural gas per year, which said was a "significant amount".
Ukraine, also heavily reliant on Russian natural gas, is trying to diversify natural gas suppliers to guarantee its energy security following Moscow's 2014 annexation of Crimea and a pro-Russian separatist conflict in eastern Ukraine.
- 'Closer' to EU, NATO -
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The controversial 11-billion-euro ($12-billion) Nord Stream 2 energy link between Russia and Germany is to run under the Baltic Sea and is set to double Russian gas shipments to Germany, the EU's biggest economy.
Kiev, Warsaw and Washington oppose the pipeline, fearing it will increase Europe's reliance on Russian energy supplies which Moscow could then use to exert political pressure.
Speaking alongside Zelensky, President Duda said Poland wants neighbour "Ukraine to be closer to the European Union, to be closer to NATO" and reaffirmed calls for the return of Crimea.
Poland, which believes its security depends on Ukraine remaining outside of Russia's sphere of influence, has long been a vocal advocate of closer ties between Kiev and the West.
Duda hailed Zelensky, a 41-year-old comedian who scored a surprise victory in April elections, as a "president with an open mind and who wants his country to be part of the free world, the democratic world".
Zelensky said he and Duda "spoke very honestly about many existing problems", adding there were "no matters that we can't resolve with dialogue and understanding".