The 19-year-old sent her first tweets the same day she finished school, which she said was a "bittersweet" moment.
She said her thoughts turned to the millions of girls who would never get the same opportunity.
Malala was just 11 when she first began writing a blog on girls' education.
But it was after she was attacked as she boarded her school bus in October 2012, aged 15, that her fight made global headlines.
Malala was flown to the UK to undergo urgent treatment to save her life. She recovered, and has been attending school in the UK ever since.
On Friday afternoon, like millions of teens before her, she took her first steps into the world of social media, using the Twitter handle @Malala.
As well as calling for people to back her fight for girls' education, Malala also revealed she would be spending the summer campaigning around the world.
It was reported earlier this year she had received a conditional offer to study at a top UK university.
Malala Yousafzai, the Pakistani campaigner who survived being shot in the head by Taliban gunmen, has joined Twitter with a call for people to help her fight for girls' education.