She called for unity in the face of the attack and expressed sympathy for the families of the victims.
It is believed the bomb was targeted at Christian families who were in a park celebrating Easter.
“I am devastated by the senseless killing of innocent people today in Lahore,” Ms Yousafzai said in a statement.
“My heart goes out the victims and their families and friends.
“I condemn this attack in the strongest possible terms.
“We stand together with the families of the victims.
“Pakistan and the world must unite. Every life is precious and must be protected.”
Ms Yousafzai was shot by the Taliban in 2012 after campaigning for the rights of girls to attend school in the Swat Valley region of Pakistan, where the militant Islamist group had influence.
She is now an international advocate for access to education and the world's youngest Nobel Prize laureate.
The Taliban splinter group Jamaat-ul-Ahra has claimed responsibility for the attack and admitted it was religiously motivated.
"The target was Christians," said Ehsanullah Ehsan, a spokesman for the group.
"We want to send this message to Prime Minister Nawaz Sharif that we have entered Lahore."
The group has been fighting an insurgency and aims to overthrow the government and establish Islamic law.
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