The wicketkeeper flicked a four through mid-wicket to snatch victory in the final over and spark wild celebrations from a club record crowd of 11,000 at the Brightside Ground.
Needing 14 from the final over, a huge six over long off by Andrew Tye helped steady the nerves – and with three needed from two balls, Roderick (22 not out) caressed the ball through the vacant mid-wicket area to seal the win.
Earlier, Somerset had limped to 158-5 from their 20 overs, despite a quickfire 40 from 26 balls from West Indian opener, Chris Gayle.
In reply, Michael Klinger struck 60 from 43 balls before being bowled by Lewis Gregory.
This brought Andrew Tye in to join Roderick and the pair held their nerve to see Gloucestershire to a famous victory off the penultimate ball.