The former party president beat Norman Lamb in the contest to replace Nick Clegg, with 56.5 per cent of the votes cast.
The Westmorland and Lonsdale MP is to make his first public appearance as leader at a central London rally.
"Our job now is to turn millions of liberals throughout the UK into Liberal Democrats", he tweeted.
Norman Lamb offered his congratulations, saying Mr Farron would make a "fantastic" leader who championed social justice.
Nick Clegg tweeted: "It's a tough job but the best in politics. I know Tim will do us proud."
Farron now faces a monumental task to rebuild the Liberal Democrats after a disastrous general election that removed 49 party MPs from the Commons, leaving the party with just eight MPs.