Poroshenko posted this on his Twitter page.
"Shocked by and condemn dreadful terror attack in Westminster. My thoughts are with the victims of this horrible act and their families," the President of Ukraine wrote.
Five people, including a police officer and an attacker, died and 40 others were injured in the attack at the Palace of Westminster and Westminster Bridge in London, the UK.
The attack unfolded at about 14.40 GMT when a single attacker drove a grey Hyundai i40 along a pavement over Westminster Bridge, near the Houses of Parliament in central London.
PM Theresa May said the attack was "sick and depraved" and struck at values of liberty, democracy and freedom of speech.
The attacker has not been named by police.