Citing family pressures as well as political ones, Ms Davidson said it was with a "heavy heart" that she had decided to step down after "the privilege" of leading her party for eight years. She will serve as a backbench MSP until the 2021 Holyrood elections but has made no decision on whether she will run for Holyrood at that time.
In a packed press conference at the McDonald Holyrood Hotel in Edinburgh, it was revealed that Ms Davidson broke the news of her resignation to Boris Johnson in a phone call yesterday.
There had been growing speculation that a fraught relationship was putting intolerable pressure on the Scots Tory leader. She had not backed him to replace Theresa May as Prime Minister and had in the past had admitted to having "personal concerns" about him. She was also said to have been "livid" when he sacked her close ally and former Scottish Secretary David Mundell, from his Cabinet.
Ms Davidson has also been a vocal critic of Mr Johnson's "do or die" approach to Brexit, but today she said she was assured that he wanted to get a deal.
And a party spokesman said that her decision was not as a result of the Prime Minister's decision to suspend Parliament, claiming she had been considering her position for around a year, and more so after the European elections.
Today Ms Davidson was adamant that she supported Mr Johnson, saying: "I stand four-square behind the PM attempts to bring back a deal to the Commons.
"To colleagues who want to avoid no deal, vote for a deal, you've had three opportunities, don't ruin a fourth. When the PM brings back a deal to the Commons, for god's sake get behind it and this time, at the fourth time of asking, vote for it."
Ms Davidson said that she had met with the Prime Minister in London last week and had "stared him in the eye" and demanded an assurance that he was attempting to get a deal.
"I asked him outright are you trying to get a deal or not? He categorically assured me that he was, that he believes his efforts in Biarritz [at the G7 summit] opened the door a crack.
"What would help further is for those who don't want no-deal is to back the deal this time."
She said that "while a lot had been written" about her relationship with Mr Johnson, as well as speaking to him yesterday she had written a formal letter in which she "personally thanked him for his commitment to facing down another second independence referendum and securing Scotland's place in the union."
She said she also thanked him for his "positivity, enthusiasm and his understanding of Scotland".
The Prime Minister later said that he thanked Ruth Davidson on "behalf of everyone in the Conservative and Unionist Party for her wonderful service over the last eight years.
“She has dedicated so much time and energy to the role and has been instrumental in the revival of our electoral fortunes in Scotland.
“She should take immeasurable pride in the pivotal role she played during the Scottish independence referendum campaign, where her passionate support for our Union was crucial to the result.
“I will miss the incredible leadership she has given to our party in Scotland, but I am delighted that she will continue to use her enthusiasm, passion and dedication to make the case for Scotland’s place in the United Kingdom from the backbenches and beyond.
“I wish Ruth, Jen and Finn all the best for the future.”
Standing behind a podium in her trademark black trouser suit, Ms Davidson read out her resignation letter, which she had posted on Twitter a short time before.
She said it had "been the privilege of my life to serve as Scottish Conservative leader. This morning I wrote to the Scottish party chairman to tender my resignation".
She added: "While I have not hidden the conflict I have felt over Brexit, I have attempted to chart a course for our party which recognises and respects the referendum result."
Ms Davidson's leadership had seen a revival of Tory fortunes in Scotland, as they more than doubled their number of seats at Holyrood in the 2016 election - making the party the second biggest behind the SNP - and then increased its MPs from one to 13 in the 2017 general election.
In her resignation statement she said her party's electoral successes had paled in significance compared to the role it had played in the campaign to keep Scotland in the UK during the independence referendum campaign.
She added: "It was - without doubt - the most important contribution of my working life and, I believe, will remain so."
But she said the biggest change over the years had been the birth of her son and as a result the thought of spending hours campaigning in a potential General Election now filled her "with dread".
She said: "As I look to the future, I see the Scottish election due in 2021 and a credible threat from our opponents to force a general election before then. Having led our party through seven national elections and two referenda, I know the efforts, hours and travel required to fight such campaigns successfully.
"I have to be honest that where the idea of getting on the road to fight two elections in 20 months would once have fired me up, the threat of spending hundreds of hours away from my home and family now fills me with dread. That is no way to lead."
Ms Davidson gave birth to her, and her partner Jen's, son Finn in October last year. The pair are now planning to get married.
She said she feared her focus on being a "good leader" in recent years had made her a "poor daughter, sister, partner and friend", and that the arrival of Finn "means I now make a different choice".
She added: "Be assured I will continue to support the party, the Prime Minister and Scotland's place in the United Kingdom from the backbenches and beyond," she said.
First Minister Nicola Sturgeon took to Twitter shortly after the announcement.
She said: "I wish Ruth well for the future. I know well the toll political leadership can take on family life, and no one will grudge her more time with her young son.
"There will be opportunity later to discuss the politics of her decision - but for now, I’ll simply send my best wishes."
And new Scottish Secretary Alister Jack thanked her for her "outstanding" work as leader.
Mr Jack, who took over from David Mundell said: "I want to put on record my thanks for everything she has achieved. Her leadership has been inspirational and transformational.
"She has inspired a new generation of Scottish Conservative politicians, activists and supporters - and in turn that has transformed our party's fortunes. In short, she has made the Scottish Conservatives the only credible challenger to the SNP in Scotland."
He added: "It is thanks to Ruth that we have become the official opposition at Holyrood, holding Nicola Sturgeon's failing, high-tax, independence-obsessed administration to account so effectively.
"It's also thanks to Ruth that we have a strong complement of 13 Scottish Conservative MPs standing up for Scotland's interests at Westminster."
Mr Jack continued: "At the heart of Ruth's success has been her brilliance as a communicator and her passion for strengthening Scotland's place in the UK.
"She has consistently articulated a positive vision of the Union and that is why she played such an important role in the successful 2014 campaign to stay part of the UK and why a majority of Scots continue to oppose a second independence referendum.
"She is a huge loss but she steps down with an incredibly impressive record of achievement to look back on and I wish her my very wishes for the future."
A close ally of Ms Davidson, Miles Briggs, Conservative MSP for the Lothians tweeted: "I've nothing but love and pride for my friend @RuthDavidsonMSP today. She is one of the most talented politicians of her generation and still has so much to contribute to our national political life. Thanks to Jen and Finn for letting us share her for 8 years. #ThankYouRuth"
Former Prime Minister Theresa May also took to Twitter saying: "Sorry to see @RuthDavidsonMSP step down as Leader of @ScotTories. Thank you for all you've done for our party and our Union over the last 8 years, and enjoy your well-deserved family time with Jen and Finn."
Scottish Labour Leader, Richard Leonard MSP, said: "There is no doubt that Ruth Davidson has been responsible for the increase in support for her party in Scotland and her departure as Leader will be a massive setback for them.
"However, the timing can be no coincidence and shows clearly that Boris Johnson's conduct as Prime Minister isn't acceptable even to those who share his politics.
And Andrew McRae, policy chair of the Federation of Small Businesses Scotland, said: “It is more than disheartening that Scottish frontline politics is losing a straightforward, pro-business voice like Ruth Davidson’s, especially during this period of turmoil.
“She has been a staunch friend of the FSB during, and before, her tenure as leader. We fully understand and respect her reasons for stepping back from the forefront of politics, but many will miss her contribution to Scottish public life.”