Fitness brands including Reebok have cut ties with CrossFit after the company's CEO posted a tweet making light of George Floyd's death.
In reply to a public health body saying racism was a public health issue, Greg Glassman tweeted: "FLOYD-19."
He also called an affiliate "delusional" for questioning why CrossFit had been silent on the killing of Floyd by police in Minneapolis.
Mr Glassman has now apologised, saying CrossFit "will not stand for racism".
He said he had been trying to make a point about lockdowns that have been put in place to try to contain the spread of the coronavirus.
CrossFit is a company based on a fitness regimen developed by Mr Glassman, and is incorporated into gyms across the world.
These gyms are run on an affiliate model, paying the main CrossFit company for permission to use the name and regimen.
It began when the Institute for Health Metrics and Evaluation at the University of Washington tweeted that "racism is a public health issue".
In response, Mr Glassman tweeted "FLOYD-19" - a play on Covid-19, the name of the disease caused by the coronavirus.
Mr Glassman also came under fire for an email he reportedly sent to an affiliate who asked for a response from the company on anti-racist protests across the US.
"I sincerely believe the quarantine has adversely impacted your mental health," he reportedly wrote to the woman, before calling her "delusional". "You think you're more virtuous than we are. It's disgusting."
Since Mr Glassman's tweet about George Floyd, hundreds of affiliate gyms have removed CrossFit from their branding. A spreadsheet tracking all of the gyms that are rebranding currently lists 227 fitness centres distancing themselves from CrossFit.
One of these gyms, Petworth Fitness in Washington DC - formerly CrossFit Petworth - wrote on Instagram: "For a brand that has preached about being 'for all', the deafening silence on current and past issues of racism tells us all we need to know."
It added that it would donate its annual affiliate fee - $3,000 (£2,364) - to the Black Lives Matter DC and Know Your Rights anti-racist campaign groups.
Adidas AG, which owns Reebok, also issued a statement confirming it was ending its relationship with CrossFit.
"Recently, we have been in discussions regarding a new agreement, however, in light of recent events, we have made the decision to end our partnership with CrossFit HQ," the company said in a statement to AFP news agency.
In a statement posted on CrossFit's Twitter account, Mr Glassman apologised for the tweet and said he was trying to make a point about lockdowns to contain the spread of the coronavirus, which he doesn't agree with.
"I, CrossFit HQ, and the CrossFit community will not stand for racism. I made a mistake by the words I chose yesterday," he said. "My heart is deeply saddened by the pain it has caused. It was a mistake, not racist but a mistake.
"Floyd is a hero in the black community and not just a victim. I should have been sensitive to that and wasn't. I apologise for that.
"I was trying to stick it to the @IHME_UW [Institute for Health Metrics and Evaluation] for their invalidated models resulting in needless, economy-wrecking, life-wrecking lockdown, and when I saw they were announcing modelling a solution to our racial crisis, I was incredulous, angry, and overly emotional. Involving George Floyd's name in that effort was wrong."