The MP for East Dunbartonshire won the leadership contest with 47,900 votes. She is also the first female to ever be the leader of the party.
Jo Swinson has become the first female Liberal Democrat leader, after decisively beating Sir Ed Davey in a poll of party members.
She won 47,997 votes, against her opponent’s 28,021.
The 39-year old, who succeeds Sir Vince Cable, said she was “over the moon” to have been elected and was “ready for the fight of our lives”.
She told activists the UK’s future lay in the European Union and she would do “whatever it takes to stop Brexit”.
As well as being the first woman to take charge of the party, Ms Swinson is also its youngest ever leader.
Describing Boris Johnson, the frontrunner in the contest to be the next Conservative leader, as “unfit to be prime minister”, she said her party was ready to return to government.
“I stand before you today not just as leader of the Lib Dems, but as a candidate to be prime minister. There is no limit for my ambition for our party, our movement and our country.
“I am ready to take my party into a general election and win it.”
The BBC’s political correspondent Jessica Parker said Ms Swinson’s victory speech was met with rapturous applause.