However, Deputy First Minister Nicola Sturgeon ruled out a name change to Robert Burns International, to avoid confusion.
She told MSPs the extra funding was needed to clear a maintenance backlog and refurbish the terminal, including the duty-free shop, so passengers spent more money.
It brings the total ploughed into the Ayrshire airport to some £15m, all in loans, which must be repaid with interest.
The Scottish Government bought the airport in November for £1 to avert closure, following a steep decline in passenger numbers and annual losses of some £7m.
Ms Sturgeon said the additional funding would comprise £4.3m to clear a backlog of “essential maintenance”, £3m for operational support and £2.4m in “repositioning capital”.
Critics expressed alarm at the news. Glasgow City Council’s Labour leader Gordon Matheson said: “I have serious concerns about how the Scottish Government can justify using up to £15m of taxpayers’ money to support an airport without a viable business case, while not contravening state aid rules.”
Glasgow Chamber of Commerce chief executive Stuart Patrick said: “We will continue to raise concerns regarding the level of passenger support for Prestwick and its impact on the ability of Glasgow airport to compete on new routes.”
A spokesman for Edinburgh airport said: “£15m spent on supporting Prestwick airport is a significant figure and we believe that money could be more effectively used elsewhere in a way that would create sustainable jobs and economic benefits. The figure is five times more that Scotland currently spends on marketing itself internationally.”
A Glasgow airport spokesman said: “We have previously received assurances that Glasgow airport will not be placed at a competitive disadvantage, but will continue to seek clarity on how the government intends to develop its asset.”
Labour infrastructure spokesman James Kelly called for more details of the long-term plan for the airport to be published.
Ms Sturgeon said there was “no quick fix” for the airport and it would remain loss-making for several years.
Ryanair is Prestwick’s sole passenger airline, with 24 routes. The airport also handles freight, maintenance and training.
Ms Sturgeon said there were “strong commercial reasons” for retaining the airport’s name.
She added: “Not everyone would know of Burns’ connection with Prestwick.”
A spokesman for the Scottish Government’s Transport Scotland agency said: “All funding being provided to Glasgow Prestwick airport is being made on a commercial basis. This loan funding will attract a market rate of interest in line with state aid rules.”