The Black Eyed Peas star was cheered on by thousands of people as he ran through Taunton carrying the flame.
The United States-born music producer was a surprise addition to the line-up on day three of the relay.
Will.i.am, 37, looked at ease in the warm weather as he jogged through the Somerset town centre.
The Metropolitan Police Torch Security Team, which runs alongside the torchbearer, kept a watchful eye on the cheering crowd.
The music producer’s torch was ‘kissed’ by that held by 26-year-old taekwondo instructor Chloe Lock, from Honiton, Devon, to light the flame.