First announced last month, 'Rally Expansion Pack' is set to provide an authentic rally experience over new surface types, rally-tuned cars and the addition of a co-driver.
It will be available from December 18 for 1,600 Microsoft Points (£13.70/$20), or free to Season Pass holders.
Elsewhere, the first round of content for Season Pass holders has been revealed, including the first monthly car pack and exclusive content.
The first car pack will feature six new vehicles: the 2012 Ferrari 599XX Evoluzione, 012 Lamborghini Aventador J, 2012 Gumpert Apollo Enraged, 2012 Alfa Romeo 8C Spider, 2012 Ford Mustang Boss 302 and 1957 Chevrolet Bel Air.
Six monthly car packs are planned in total, each costing 400 Microsoft Points (£3.60/$5), or free as part of the Season Pass.
Meanwhile, Season Pass owners will receive five cars delivered monthly for the next five months, the first of which is the 2006 Lamborghini Miura Concept in November.
Forza Horizon will be available on October 26 in Europe and October 23 in North America.
> Read our review of Forza Horizon, out October 26
Watch the launch trailer for Forza Horizon below: