The new gearbox system, which includes optional steering-wheel paddles, is available to UK buyers for both the 1.4-litre MultiAir petrol engine and the 2.0-litre engine, both of which return 170bhp.
Alfa says the new TCT transmission improves fuel economy in both models but does not affect performance of the sporty five-door hatchback.
The TCT (Twin Clutch Transmission) is designed to be lighter, cost less and to handle more power but remain smoother than a standard automatic or manual transmission. It also works alongside fuel-saving technology such as Start-Stop.
The petrol version, which uses a former International Engine of the Year, features CO2 emissions of 121g/km. This puts it into Band C of road tax in the UK – enough to make it free for the first year.
Fuel economy in the petrol model is an impressive 54.3mpg while performance figures include a 0-62mph time of 7.7 seconds.
Meanwhile, the diesel engine offers Band B levels of CO2 emissions (119g/km) but a slightly slower 0-62mph time of eight seconds.
It also boosts fuel economy to 62.8mpg, making it the most frugal model in the new Alfa Romeo Giulietta range.
The Alfa TCT transmission for the Giulietta was first introduced last year at the Paris Motor Show but it has only been made available for UK customers with both petrol and diesel engines this month.
The new Alfa Romeo TCT gearbox first debuted on the smaller Alfa MiTo earlier in the year, again on the 1.4-litre MultiAir engine.
The new Alfa Romeo Giulietta with the TCT gearbox starts from £21,855. It is available on both engines and with Lusso or Veloce specification levels.