The 33-year-old leg spinner is Sussex Sharks’ leading wicket taker in the format, with 106 wickets in 131 appearances over the past 14 seasons.
Beer said: “I had hoped to carry on playing in all formats, but I am still grateful to have the opportunity to continue my cricketing career at Sussex with this T20 contract.
“My desire and drive to win trophies with the club where I’ve been since I was ten remains as strong as ever.
"This deal allows me to focus all my efforts on T20 and be the best player in that format I possibly can to give the team the best chance of winning games and ultimately the title.
“I’ll also have the time and opportunity now to think about other ventures, both for life after cricket but also in the numerous tournaments around the world that take place all year round.
“I’m looking at this next chapter in my life and career really positively and want to do everything I can to bring more success to the Sharks in T20 cricket.”
Sussex’s T20 head coach, James Kirtley added: “Will has been an ever present in the T20 squad for over a decade and is the leading T20 wicket taker at the club. To have his experience in our T20 squad is very important.
“His knowledge of the game and the information he passes on is of a very high standard and he’s shown himself to be a reliable performer for a long time.
"I am sure Beero is keen to continue to deliver at a high level on and off the pitch.”