In the new role, Tremlett will spearhead an aggressive programme of new store acquisitions and refurbishments for the society, which has more than 100 stores across Hampshire, Berkshire, Dorset, Somerset, the Isle of Wight, West Sussex and Wiltshire.
He will also oversee the rebranding of Southern Co-op's estate to The Co-operative Group's The Co-operative fascia.
The society announced in February it would replace its own grey fascia with the national brand this year.
"I am delighted to join a progressive organisation that has developed class-leading small store formats," said Tremlett, who was at Somerfield for nine years, during which he was responsible for store design, branding, space planning and location planning. He also worked on the roll-out of the retailer's Market Fresh format, which resulted in a 20% increase in sales in Somerfield stores converted to the format.
Prior to Somerfield, Tremlett held strategic development roles at Safeway, Woolworths, WHSmith and retail jeweller Signet Group.
Tremlett's appointment comes as Southern Co-operatives prepares to increase its estate through franchising.
The society is understood to have recruited more than 20 independent retailers to run its stores and is eyeing as many as 100 franchise stores in the future.