MacDonald’s move, which is subject to international clearance, means a second spell in the Scottish Premier League for the 20-year-old.
The former Burnley youth team player of the year spent a spell on loan with Falkirk last season and heads north again for the chance of first-team football.
MacDonald, who won his first cap at Scotland under-21 level last December, is top scorer for the Clarets’ reserves but has not been able to add to his eight first-team appearances this season.
Caley boss Terry Butcher, whose team is fifth in the SPL, said: “We’ve had a look at Alex in training and he’s shaped up very well.
“We’re pleased he’s agreed to see out the season with us and he adds to our attacking resources.”