The 26-year-old joins teammates Ryan Mills and Sam Lewis in committing himself to the Club ahead of the 2018/19 season.
Annett, who joined Warriors in 2014, played a key role in his first season at the Club, making 22 appearances, as Warriors secured promotion back to the Aviva Premiership at the first time of asking.
Annett was also part of the squad which won the British & Irish Cup that season, and also helped Worcester Cavaliers reach their first ever A League final.
He then played in all but one of Warriors’ 22 Premiership games back in the top flight in 2015/16 as he cemented his spot in the first-team.
Last season saw the former Ulster man take his appearance tally for the Club to 52 before unfortunate injuries kept him sidelined. However, Annett is set to return to action in April once he recovers from a knee problem.
Annett said: “It’s been a frustrating year or so for me with regards to injuries, especially after a great couple of seasons prior to that.
“But I’d like to go on record and thank the Club for looking after me during the past year or so during a difficult time.
“We’ve got a cracking bunch of lads here and hopefully if we can keep everyone together then we can push on up that table.
“For me now I just cannot wait to get back on the field and I’m doing everything off it to ensure I come back in the best place I possibly can.”
Warriors Director of Rugby Alan Solomons added: “Niall has had an unfortunate 12 months but we can’t wait to see him fit and back on the field.
“He has shown tremendous work-rate in the past few seasons at the Club and has been a very important member of the side.”