President Attending Poroshenko Inauguration
Pahor's attendance is "an expression of support for the territorial integrity and sovereignty of Ukraine and the efforts of the new president to secure democracy, peace and prosperity," his office has said.
Other senior guests at the ceremony in Kiev include US Vice President Joe Biden, European Council President Herman Van Rompuy and German President Joachim Gauck, while Russia is to be represented by its ambassador.
Poroshenko, a billionaire businessman and politician, won the 25 May presidential election in Ukraine with 54.7% of the vote ahead of Yulya Tymoshenko, the former prime minister.
The early vote was called by the interim government in the wake of the fall of President Viktor Yanukovych. Yanukovych's ousting following protests prompted Russia to annex Crimea, encouraging separatist sentiment in eastern Ukraine.
Slovenia has backed Ukraine's sovereignty and territorial integrity and condemned Russia's move as illegal, while joining EU targeted sanctions against Russian individuals and two companies from Crimea.
But the country has been against an expansion in sanctions, and has been calling for dialogue to find a political solution to the crisis.