Nicola Sturgeon has responded to as finance secretary amid claims he sent messages to a 16-year-old boy.
The First Minister said Mackay 'failed to meet the standards required' and accepted his resignation this morning.
The Scottish Sun claims he contacted the boy over a six-month period on Instagram and Facebook.
The newspaper claims that the 42-year-old MSP for Renfrewshire North and West invited the boy to dinner and to attend a rugby event.
Nicola Sturgeon said: “Derek has taken full responsibility for his actions and apologises unreservedly for them to the individual and to those he has let down. He has submitted his resignation as a government minister which I have accepted.
“Derek has made a significant contribution to government, however he recognises that his behaviour has failed to meet the standards required.
“The Minister for Public Finance, Kate Forbes, will present today’s Scottish Government budget setting out our actions to support the economy, back our people and public services and tackle the climate crisis, and that remains my government’s focus.”
In a statement Derek McKay said: "I take full responsibility for my actions. I have behaved foolishly and I am truly sorry.
"I apologise unreservedly to the individual involved and his family. I spoke last night to the First Minister and tendered my resignation with immediate effect.
"Serving in government has been a huge privilege and I am sorry to have let colleagues and supporters down."
Scottish Conservative interim leader Jackson Carlaw has also called on Mackay to resign as an MSP.
Mackay was due to present the Scottish government's budget at Holyrood later, with Public Finance Minister Kate Forbes now stepping in in his place.
