Academy-graduate Cox, 24, had one year left on his existing deal but has now agreed to stay at New Road until at least 2020.
"I'm a Worcester boy through and through - to sign for another four years is brilliant," Cox said.
"I've been here since I was nine-years-old and never thought of leaving. I want to play my cricket here."
Cox famously made his debut for the club as a 17-year-old schoolboy in September 2009 against Somerset at Taunton.
He scored 61 and claimed former England opener Marcus Trescothick as his first victim behind the stumps.
Cox's tally of 778 runs - including a maiden hundred and five half-centuries - was Worcestershire's second-highest as the county were relegated from Division One of the County Championship in 2015.
"To have somebody of his loyalty towards the club, and passion for the club, to reward him with a long contract is exactly what we are all about." director or cricket Steve Rhodes said.
"He's a terrific cricketer now, as consistent a wicketkeeper in county cricket that I have seen and to do that at a young age is incredibly good.
"But also his match-winning experience and ability with the bat in the different formats has been excellent."
Cox is the 10th player to have signed a new long-term contract at Worcestershire in the last 10 months.
Wicketkeeper-batsman Ben Cox has signed a new four-year contract with Worcestershire.