The former Arsenal and England midfielder was a free agent after his West Ham contract was terminated last October.
Wilshere spent the 2016-2017 season on loan with the Cherries and has been handed a contract, having been invited to train with Jason Tindall's promotion hopeful.
The 29-year-old, who has been badly blighted by injuries in his career, told afcbTV: "It feels great to be back and I can't wait to get started.
"From the minute I walked through the door three weeks ago, it felt natural and like I had never been away.
"I know a lot of the players and staff here and I'm pleased they allowed me to train initially, and then it just went from there.
"I've been impressed by the coaching staff, the way they work and the ambitions of the club to get back in the Premier League.
“𝙄𝙩 𝙛𝙚𝙚𝙡𝙨 𝙫𝙚𝙧𝙮 𝙜𝙤𝙤𝙙. 𝙁𝙧𝙤𝙢 𝙩𝙝𝙚 𝙢𝙞𝙣𝙪𝙩𝙚 𝙩𝙝𝙖𝙩 𝙄 𝙬𝙖𝙡𝙠𝙚𝙙 𝙩𝙝𝙧𝙤𝙪𝙜𝙝 𝙩𝙝𝙚 𝙙𝙤𝙤𝙧 𝙩𝙝𝙧𝙚𝙚 𝙬𝙚𝙚𝙠𝙨 𝙖𝙜𝙤, 𝙞𝙩 𝙛𝙚𝙡𝙩 𝙣𝙖𝙩𝙪𝙧𝙖𝙡.”@JackWilshere is raring to go 👊— AFC Bournemouth 🍒 (@afcbournemouth) January 18, 2021
"We all share the same ambitions and I'm looking forward to playing my part in helping achieve those."
Bournemouth is third in the Championship, eight points adrift of leader Norwich City after being relegated from the Premier League last season.