In the first three months of year, Police Scotland had the equivalent of 17,170 full time officers, according to Scottish Government statistics.
The last time the number of officers was lower was in the first three months of 2009, when the total was 17,048.
Officer numbers increased after the SNP came into power in 2007, with the party committed to putting 1,000 extra police on the streets.
That commitment was first met in March to June 2009, with the number of officers having remained at above or about 17,250 since then.
But the total for January to March this year dropped by 86 from the 17,256 recorded in October to December 2017.
Despite the fall in officer numbers, justice secretary Michael Matheson stressed frontline policing was being maintained, with staff being freed up from back office roles.
The commitment to having 1,000 more officers than in 2007 has not applied since the ten-year Policing 2026 strategy was introduced in 2016.
However, Mr Matheson said Scotland still had “significantly” more police than when the SNP came to power in 2007 – with 16,265 full time equivalent officers in April to June that year.
The justice secretary stated: “The Policing 2026 strategy committed Police Scotland to the right workforce mix, including civilian specialists and support staff to tackle the emerging challenges of modern criminal activity while keeping officer numbers significantly above what this government inherited in 2007. Today’s figures confirm this plan is taking effect and they have signalled that a number of officers have been freed up from back-office roles to ensure the frontline is maintained and enhanced.
“While staffing is a matter for the chief constable, I am acutely aware that the public will want to be assured that our hard working police officers are able to offer the best possible service to keep our communities safe.
“HMICS and the Scottish Police Authority must continue to oversee the implementation of the strategy to ensure the force can continue to provide an excellent service across Scotland.”
Opposition politicians reacted with concern to fall in officer numbers, with Conservative justice spokesman Liam Kerr describing the drop as “massively disappointing”.
He said: “The SNP have demonstrated time and again that their priorities lie elsewhere,”
Labour’s Daniel Johnson said: “For the first time, the SNP government has admitted it has failed to maintain its flagship pledge of providing an ‘extra’ 1,000 police officers.
“SNP ministers’ focus on spin over substance is finally coing undone – we all know that ‘extra’ officers were simply backfilling for a loss in back room police staff.”