The Saints narrowly saved themselves from getting relegated last season, and the 28-year-old played a huge part in their survival battle.
McCarthy joined the Saints from Crystal Palace in 2016, and struggled to get regular games. However, he replaced the error-prone Fraser Forster in January, and made a strong impact.
The former Crystal Palace star started only 23 games, and was named the south coast side’s Player of the Year, such was his influence in the side.