Chevron said the Alder project has already created hundreds of jobs across the UK, including in Aberdeen and Invergordon.
Gas produced from the field will be sent via a 17-mile (28km) pipeline to the ConocoPhillips-operated Britannia platform.
The Alder field was discovered in 1975.
Chevron said development was originally considered impossible due to the challenges of the field's reservoir, lying 14,700ft (4,480m) beneath the seabed.
The company said it was now viable due to the "right combination of technology, infrastructure for processing and export, and commercial conditions".
Chevron Upstream Europe managing director Greta Lydecker said: "First gas at Alder represents a significant milestone for Chevron and highlights our commitment to investing and developing resources in the UK.
"Alder supports our goal of helping maximize the economic recovery of the UK, adds significant production to our portfolio, and helps extend the field life of Britannia, an important asset to Chevron in the North Sea."
Deirdre Michie, chief executive of industry body Oil and Gas UK, said: "It shows the opportunities that are still out there."
She called for a "stable fiscal regime" to help boost investor confidence in North Sea exploration and production.
Gas production has started from a North Sea field first discovered more than 40 years ago when its development was considered "impossible".