The MSP for Renfrewshire North and West has resigned with immediate effect this morning following allegations he sent almost 300 messages to the teen.
The politician was due to present the Scottish government's budget at Holyrood later, however, Public Finance Minister Kate Forbes will now take his place.
The Scottish Sun alleged that he contacted the boy over a six-month period on social media, sending a total of 270 messages.
The newspaper claims that the 42-year-old MSP invited the boy to dinner and to attend a rugby event, and also called him "cute".
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In a statement Derek Mackay 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."
Reacting to the news, First Minister Nicola Sturgeon said his conduct "failed to meet the standards required".
The Paisley Daily Express has contacted Derek Mackay's office for comment.
More to follow.
