"No pardon law will excuse the terrorists who organised the attack [on] the innocent victims of MH17," Poroshenko said in Brussels, after talks with EU leaders.
"We [will] never, ever pardon them, and they should be punished, and it will be under the courts."
He said he delivered his message to Dutch Prime Minister Mark Rutte. The majority of the 298 people aboard the airliner, which was shot down July 17 over eastern Ukraine, were Dutch citizens.
Russian-backed Ukrainian rebels are suspected of being behind the incident, though it is not known who was responsible.
Dutch foreign minister Bert Koenders said justice must be served, while the parliament in The Hague has demanded a guarantee that criminal investigations continue unhindered.
Relatives of the victims had insisted that "those responsible for the crash and therefore for the murder of 298 innocent passengers be investigated, pursued and punished".