The winger, who joined the Shrimpers at the age of 15 following his release by Arsenal, has signed a two-year deal.
Pinnock made his debut as a late substitute in the 2-0 win at Aldershot in mid-April before enjoying 25 minutes of the club's final-day defeat at home to Morecambe.
"I'm delighted to have signed my first professional contract here at Southend, I'm just now looking forward to kicking on and making an impression this season," he told the club's official website.
"I'd like to thank all the staff in the youth department who have helped me to develop my game over the years and earn this contract.
"Having that little taste of what the first team is like towards the end of the season has made me even hungrier for success. I'm already looking forward to the start of pre-season."
- Sports & Recreation