Alun Griffiths will begin work in March 2019 and is expected to take two years to complete.
The upgrade involves putting in traffic signals and enlarging the existing roundabout, widening of the eastern exit and building a new roundabout to access the proposed Nexus 25 industrial park.
Work involves the construction of a new 250-metre long dual two-lane access road and 250 metres of new single carriageway. The contractor will also reconstruct 200 metres of existing dual carriageway and build a new bridge over Blackbrook.
Alun Griffiths director Martyn Evans said: “We are delighted to be appointed to deliver this important scheme with Somerset County Council. Griffiths have, over many years, developed a close working relationship with the county resulting in the successful delivery of significant projects which have provided added value to the local economy.
“We look forward to starting work on Junction 25 and recognise the importance of this scheme for the local community and route users. Griffiths will work considerately with the community and do our best to minimise disruption to road users during the various construction phases of the programme.”
The J25 scheme is expected to cost around £19.2m in all, with allocated funding from the Heart of the South West Local Enterprise Partnership (LEP) Growth Deal Fund, Taunton Deane Borough Council, Highways England, Somerset County Council and developer contributions.