Kezia Dugdale has resigned as Scottish Labour leader
The 36-year-old informed Labour MSPs of her decision via a WhatsApp message.
Kezia Dugdale has resigned as leader of the Scottish Labour Party.
The 36-year-old had led the party since 2015.
STV News understands most of the party's MSPs were informed of her decision in a WhatsApp message.
Dugdale informed the party's chairwoman, Linda Stewart, in a letter earlier on Tuesday.
She said: "As chair of the party, I am writing to you today to resign as leader of the Scottish Labour Party.
"It has been an honour and a privilege to have served this party in a leadership position for the last two and a half years, covering four national elections and one referendum."
The MSP added: "But with nearly four years now until the next Scottish Parliament elections, I am convinced that the party needs a new leader with fresh energy, drive and a new mandate to take the party into that contest.
"I will continue as a Labour MSP for the Lothians and am already looking forward to spending more time with constituents and on constituency issues."
UK Labour leader Jeremy Corbyn thanked Dugdale for her work in rebuilding the party in Scotland.
He tweeted saying "Kezia became the Scottish leader at one of the most difficult times in the history of the Scottish Labour Party, and the party's revival is now fully under way, with six new MPs and many more to come".
SNP leader Nicola Sturgeon tweeted: "We may be opponents, but @kezdugdale led her party with guts and determination and I admired her for that. I wish her well for the future."
Leader of the Scottish Conservatives Ruth Davidson wished Dugdale well and said she "deserves the thanks of her party for putting in the hard yards".
Scottish Lib Dem leader Willie Rennie said Dugdale had been "good for Labour", while Scottish Greens co-convener Patrick Harvie said she had "served her party with dedication".