During the closing stages of the race, Magnussen, Alonso, Sebastian Vettel and Jenson Button were involved a toe to toe battle for fifth place.
On the straight following turn four, Alonso tried to overtake the Dane but the Spaniard was squeezed onto the grass after Magnussen didn't leave him enough space.
"The driver of car 20 [Magnussen] was defending his position on the straight between turns 4 and 5, a significant portion of car 14 [Alonso] was alongside car 20," a statement from the steward reads.
"The driver of car 20 did not leave enough space for car 14 and forced the car off track."
As a result of the 20-second penalty, he drops down from sixth place to 12th, while Nico Hulkenberg moves up to 10th and will pick up the final point.
Magnussen has now accrued four penalty points.