Bank Santander took out the policy which would pay out if anything happened to the driver's digits meaning he could not race.
A spokesperson for the bank said: "Alonso's thumbs are a symbol, as well as being essential in Driving a Formula 1 car, are a sign of victory and that everything is under control."
The announcement that each thumb is insured for £4.5m took place in Madrid ahead of next week's Spanish Grand Prix in Barcelona.
Alonso will now also be sponsored by Santander's insurance branch… and will soon get bored of giving people a thumbs-up.