The 20-year-old, who was part of Australia's success over Indonesia in October to qualify for next year's Under-23 Asian Cup in Uzbekistan, is returning to Coopers Stadium after being with the Reds before his move to England in 2020.
Brook has been part of Brentford's 'B' side and is looking forward to showing how much he's developed as a player since his previous stint at Adelaide.
"So far, my time in England and training with an English Premier League side has really helped evolve my game and I can't wait to bring that back to the Reds," Brook said.
Brook's arrival comes after the exit of winger Ben Halloran earlier this week to South Korean top-flight club FC Seoul.
Adelaide coach Carl Veart said Brook will go some way to boost the Reds' attack in Halloran's absence.
"Having been training day-in, day-out with a team playing in what's arguably the strongest football league in the world, I'm sure Lachlan has improved immensely," Veart said.
"We're very much looking forward to seeing how much he's developed throughout this season."
Adelaide captain Stefan Mauk is also set to be on the move in January, confirming he's in talks with an unnamed Japanese club but the midfielder is expected to face A-League Men champions Melbourne City on Saturday.
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