Poroshenko was elected in 2014 after a pro-Western government took over from the pro-Russian president who fled the country following months of protests.
Shortly after Russia annexed Ukraine's Crimean peninsula and threw its weight behind separatist rebels in the east.
Russian officials have claimed the new Kiev government would have turned Crimea, home to a Russia-leased naval base, into a NATO base.
Speaking at a meeting Monday with NATO Secretary General Jens Stoltenberg in Kiev , Poroshenko said Ukraine would not be applying for a NATO membership "immediately" but would instead "build a genuine program of reforms" to meet NATO requirements for membership in the future.