Hawthorn and Essendon locked in a deal for the 31-year-old on Monday afternoon, hours after the AFL trade window opened.
The Hawks will give up picks No.15 and No.25 in this year's draft, plus a 2028 second-round pick, in exchange for the seven-time Bombers best and fairest and three-time All Australian.
The midfielder joins Hawthorn on a long-term contract, reportedly worth $4 million over four years with a trigger for a fifth season.
"Zach has long been celebrated as one of the most elite midfielders in the AFL, and he will play a key role in our side for many years to come," Hawthorn list boss Mark McKenzie said.
"His professionalism and competitiveness are second to none."
In 2025, Essendon knocked back Hawthorn's final offer of two first-round picks (10 and 22), a future first-rounder, and now-delisted midfielder Henry Hustwaite.
Bombers president Andrew Welsh had warned Hawthorn not to get "too cute with us again" in an interview with the Herald Sun.
McKenzie said the Hawks had learned after waiting until about 30 hours before the 2025 trade deadline to make first contact with Essendon about Merrett.
Merrett, who was entering the final year of his contract, stepped down as Essendon captain but featured in all 23 matches on his way to securing a seventh Crichton Medal.
Essendon paid tribute to the 274-gamer as the defining Bomber of his generation.
"When people look back on this era of Essendon, Zach's name will be one of the first they mention," Essendon chief executive Tim Roberts said.
With the Merrett deal done, Hawthorn's attention will turn to acquiring Gold Coast key forward Ben King as a restricted free agent.
The Hawks are expected to lodge paperwork for King on Tuesday, with the Suns afforded three days to match their contract offer.
Gold Coast list boss Kall Burns said the Suns would take their time to consider whether to match or accept band one compensation, currently valued at pick No.7.
"You've got to weigh up what you can possibly get in if it's a trade," he said.
"It is only day one of the trade period."
Hawthorn fell at the preliminary final hurdle for a second straight year, losing to eventual premiers Brisbane by nine points.
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