The deal also includes the prestigious Macau GP and Korean Superprix, which take place in November.
Fortec enjoyed a very strong 2010 season, finishing third in the championship with rookie driver Oli Webb.
Team owner Richard Dutton commented: "I am really pleased to have Will back with us at Fortec. He is a true racer with excellent car control and certainly will be a championship contender next year."
Will added: "We had several offers on the table but this is the right decision for me. The Fortec car is very competitive and I have a good relationship with all the guys on the team. I have learned a lot this year and we are now in a good situation to plan our title bid for next year."
His father, Alf, said: "This is the logical step for us. We feel Fortec have had the car of the championship this year. The engineer was a major factor in our decision and the dynamic between Will and Fortec engineer Andi Scott is a strong combination. The Mercedes engine has proved to be very strong and completes the package we have worked hard to put together for 2011."
Andi Scott, Will Buller's new race engineer, said: "I am looking forward to an exciting year with Will. He has got stronger as the season has gone on, finishing a superb second in the season finale at Brands Hatch.
"It will be extra special to work with him as we are both from Northern Ireland and want to do our country proud."
Fortec will now kick off their winter test campaign in preparation for the upcoming Macau GP, Korean Superprix and 2011 championship.
Buller makes his move to Fortec a year after signing for Hitech following two years of competing in Formula BMW.
He has already displayed race-winning pedigree in Formula 3 machinery with victory at Interlagos in Brazil that winter.
Despite being just 18, the rising star heads into the season as one to watch and with his focus firmly on the championship crown.