The justice secretary told MSPs that Strathclyde, Tayside and Northern had officers who were routinely armed.
He said Police Scotland had adopted the approach across the country since its launch in April last year.
Lib Dem justice spokeswoman Alison MacInnes said Holyrood should have been told of the routine arming of police.
A political row over specially trained officers routinely carrying side arms started earlier this month when Independent Highlands and Islands MSP, John Finnie, raised concerns.
Mr Finnie said there had been a change of policy from firearms officers having to retrieve their weapons from locked safes in armed response vehicles with permission from a senior officer.
He said there had been occasions when firearms officers had supported unarmed police on routine duties, such as dispersing late night crowds from outside pubs and clubs.
Mr MacAskill told the Scottish Parliament that Police Scotland took a decision to follow an example previously set by Strathclyde, Tayside and Northern Constabulary.
He said it was necessary for trained firearms officers to be readily available to respond quickly to "urgent and unexpected threats".
Mr MacAskill said Police Scotland has 275 firearms officers - 1.6% of Police Scotland's personnel - and they were deployed on a shift pattern basis.
He added: "Consequently, only a small number will actually be deployed across our communities at any one time."
The justice secretary also said that the police authority and police investigation and review commissioner could review the deployment of firearms officers.
Ms MacInnes said the routine arming of officers represented a "substantial change of direction" and parliament should have been informed.
Police Scotland's Assistant Chief Constable Bernard Higgins told BBC Scotland: "They are police officers first and foremost and it's only right that they contribute to the policing plan in addressing the greatest concerns of the community.
"The fact that they are carrying firearms and a Taser, to this point, there has been no negative public reaction to it."
Three Scottish police forces allowed specialist officers to routinely carry guns before the launch of the new single force, Kenny MacAskill has said.