Addressing a gathering of Ukrainian Canadians in Toronto, Harper vowed to deal with the Ukraine crisis with "resolve, strength and an unblinking resistance to the enemies of freedom."
He accused Russian President Vladimir Putin of trying to undermine Ukraine's very nationhood.
Canada's relations with Russia soured over Moscow's annexation of Ukraine's Crimean peninsula in March and its subsequent intervention in eastern Ukraine.
Canadian officials said this week that a frigate taking part in a NATO exercise in the Black Sea was buzzed by Russian military jets Sunday.
Moscow has brushed off the accusation, saying Russian jets were indeed flying in the area but were behaving strictly in line with international rules.
"The people of Ukraine seek a future of engagement with the West because they seek what we have in the West... freedom, democracy, prosperity," Harper told more than 1,000 people, including hockey great Wayne Gretzky, attending the United for Ukraine gala.
"Sadly, Vladimir Putin wants to deny them this future. He seeks to destabilize and demoralize the people of Ukraine, to pull them back into a dark era of fear and dependence on an aggressive, expansionist and authoritarian Russian regime."
In announcing Poroshenko's visit, Harper promised that Ukraine will always have the friendship and the steadfast support of Canada.
Officials earlier said the Sept. 17 visit will provide an opportunity for the two leaders to discuss the crisis, with a particular focus on the steps required to restore Ukraine's territorial integrity.
Harper said that Canada will send observers to next month's parliamentary elections in Ukraine.
"They will be there to try help ensure Putin's agents do not halt the expression of the free will of the Ukrainian people."
Last month, Canada imposed a new round of sanctions and travel bans on several top Russian and Ukrainian politicians and groups with ties to Putin in co-ordination with the United States and the European Union.
Russia retaliated with its own sanctions that mainly target Canadian pork exports, which federal government figures say amounted to $563 million in 2012.
And on Thursday, the EU slapped more sanctions on Russia, limiting its access to the European financial market, hitting the country's vital oil industry, curbing high-tech exports and targeting more officials with travel bans and asset freezes.
The EU has said the sanctions could be reversed within weeks if the ceasefire in eastern Ukraine holds.
Foreign Affairs Minister John Baird has hinted that Canada won't hesitate to level additional sanctions against Russia if the truce fails.
Russia has consistently denied it is trying to destabilize Ukraine by sending arms or soldiers to help pro-Russian separatists fight government troops.
During his speech, Harper repeatedly noted Canada's close relations with Ukraine and said there are more than 1.2 million people of Ukrainian descent in the country.
"The only places in the world where there are more Ukrainians are Russia and, of course, Ukraine itself."