The Anik F2 spacecraft weighs almost six tonnes and will blast towards its geostationary orbit, above the Pacific Ocean, from French Guiana at 0043GMT.
The Canadian satellite will deliver broadband internet access and other digital services, such as e-learning and e-health projects, to North America.
It will be the 19th flight for the Ariane 5 and the first since March.
The rocket is the only launch system currently offered by its operator Arianespace, now that the Ariane 4 series has been retired.
However, contracts have been signed that will see a launch pad built at the Kourou spaceport to enable Arianespace to offer Russian Soyuz rockets to customers.
And a smaller rocket system for lighter payloads called Vega is also in development.
The Ariane 5 is currently winning back market confidence after suffering a major failure in December 2002, when a heavy-lift version of the rocket veered off course and auto-destructed four minutes into its flight.
Despite the set back and the grounding of the Ariane 5 for much of 2003, Arianespace continues to be the dominant player in the world's commercial satellite launch market.
In May, Arianespace and EADS-Space Transportation signed a contract which will see the construction of 30 Ariane 5 launch vehicles. The value of the order was about 3bn euros (£2bn).
