Santner, 24, broke his finger in only his second game for the county in 2016.
"There is some unfinished business with Mitch," director of cricket Steve Rhodes told Worcestershire's website.
"Last summer he was very unlucky. Losing Mitch was a massive blow to our hopes of qualifying. We'd had a good start and him playing all those games would have been an extra bonus for us."
The left-arm batsman and spinner, who made his Test debut in 2015, will rejoin the Rapids following June's ICC Trophy in England.
Santner added: "I really enjoyed my time at Worcester with a great bunch of lads and it was so frustrating for me to pick up that injury in the very first T20 game.
"It's fantastic that I've got the chance to come back again next summer and hopefully I can make up for lost time and make an impact with bat and ball."
In August, Worcestershire signed Australia pace bowler John Hastings as their main overseas player for the 2017 season across all formats of the game.
Worcestershire have re-signed New Zealand all-rounder Mitchell Santner for the 2017 T20 Blast competition.