Requests under Freedom of Information law by the Conservatives found there is almost £5 million in outstanding fines not included in statistics by the Scottish Court Service (SCS) because they date back more than three years.
SCS data shows for 2010/11 to 2012/13, the value of fines in arrears is £16.9 million.
But the Tories found there is also £1.47m in unpaid fines imposed before January 2008, £1.4m remains unpaid from 2008/9, and £1.9m remains from 2009/10.
Consideration is given to "archiving" fines when enforcement managers consider there is little prospect of recovery, the SCS said.
But the SCS said its archiving process "does not translate to fines being 'written off' and officers chase up archived fines if circumstances change or new information becomes available.
Tory justice spokeswoman Margaret Mitchell said: "The public will be appalled that almost a fifth of unpaid fines are hidden from public view.This is money the taxpayer is missing out on and it also means people who are being punished in court for a range of offences are not having to meet that penalty."
A Scottish Government spokeswoman said: "Since SCS took over responsibility in 2008, overall fines collection rates have improved considerably and are now at consistently high levels … Collection rates will improve further once SCS is able to directly access information held by Department of Work and Pensions (DWP).
"Scottish officials are working with the DWP to introduce a statutory provision to allow this to happen at the earliest opportunity."