The party is calling on the Scottish Government to act to relieve the economy’s reliance on such unpaid labour.
The plea came after its analysis of the 2020 annual population survey suggested that on average workers in Scotland are performing 7.7 hours of unpaid overtime a week.
Over the course of a year, that amounts to 100 million hours of unpaid overtime, Labour suggested, adding that if people were paid at the average hourly pay of £14.07 they would be collectively entitled to £1.4 billion more in wages.
Scottish Labour finance spokesperson Daniel Johnson said: "A fair day’s pay for a fair day’s work should be at the heart of our economy, but these statistics plainly show that many Scottish workers are losing out.
“We cannot have our economy so reliant on unpaid overtime.
“The people of Scotland should be receiving the full fruits of their labour, not missing out on millions of pounds.
“Today, Scottish Labour is calling for fairness to be put at the heart of our economy and for businesses, unions and government to work together to deliver a fair day’s pay for Scots.”
A Scottish Government spokeswoman said: “While employment law is reserved, we are clear that all employees must be paid fairly for the work they do.
“That is why the Scottish Government continues to support payment of, at least, the Real Living Wage to build a more resilient economy, a more equal labour market, and foster inclusive growth.
“Scotland has the highest proportion of jobs in the UK paying at least the Real Living Wage or more, and proportionately there are five times more accredited Living Wage employers in Scotland than in the rest of the UK.
“Through our Fair Work First policy the Scottish Government has consistently called for employers in all sectors to provide workers with a fair and equitable wage for the work they do along with safe and secure working environments.
“We also call for employers to promote diverse and inclusive workplace cultures where staff are engaged and have their voices heard.”