Hardwick told a packed media conference at Punt Road on Tuesday that it became apparent to him a couple of weeks ago that the time was right to stand down and let the players "listen to a different voice".
"I would rather leave too early than too late," he said.
Current assistant Andrew McQualter has been appointed as interim coach and will assume the reins for Sunday's clash with Port Adelaide.
Hardwick paid tribute to everyone associated with Richmond - a club he joined in 2010 after a distinguished playing career at Essendon and Port Adelaide and an assistant coaching stint at Hawthorn under his great friend Alastair Clarkson.
"It's a tough gig being an AFL coach but the support I've received from the majority of people has been absolutely outstanding," he said.
"The Richmond football club has been the love of my life."
Richmond president John O'Rourke paid tribute to Hardwick's legacy.
"History was created under his watch and for that we will be forever indebted," he told reporters.
Hardwick's 307-game tenure was the longest in Richmond history and will go down as one of the club's all-time greats alongside Tom Hafey, having led the Tigers to premierships in 2017, 2019 and 2020.
There will be speculation about the 50-year-old's future and whether - or when - he could coach again at another club next year or beyond.
Port Adelaide's Ken Hinkley is out of contract at the end of this season, West Coast's Adam Simpson is under mounting pressure after a horror run and Stuart Dew can hardly afford another year without September action at Gold Coast.
There is also a cloud hanging over North Melbourne, with Alastair Clarkson on indefinite leave from the club.
Hinkley, who rejected overtures from Essendon last year, was runner-up to Hardwick for the Richmond job in late 2009 before winning the Power role ahead of the 2013 season.
Hinkley and Port have repeatedly stated they have put off contract talks until at least August, though that could now change.
The Power may be keen to lock in the 56-year-old after seven straight wins that have seen the club rocket into premiership contention.
Collingwood coach Craig McRae said he was "shocked" by Hardwick's decision to quit when the news broke on Monday night.
McRae worked as an assistant under Hardwick through the 2017-2020 glory years and said his former mentor had coached with "courage" in a job that grinds people down.
"You get tired. You've got to find energy sources," McRae told Fox Footy.
"You're constantly trying to find the right balance to find the right energy to give to your playing group.
"You find the energy for your players and then you get home and you're exhausted. Your wife and family want you to find energy too."
The Tigers currently sit in 14th spot on the ladder, with just three wins and a draw from the opening 10 rounds.
