Moderators of some popular discussion threads also accused the platform of continuing to harbour racism and took action in support of the protests over the death of George Floyd, for example by curbing posting or becoming private.
Mr Ohanian, who is married to tennis champion Serena Williams, explained his decision to quit the board in a series of tweets.
"I believe resignation can actually be an act of leadership from people in power right now," he wrote. "I'm saying this as a father who needs to be able to answer his black daughter when she asks: 'What did you do?'"
In a lengthy statement later on Friday, Steve Huffman, Reddit's chief executive, said that the company would "honour" the request by Mr Ohanian that his successor be a black candidate.
He added the company would update its content moderation policies to "explicitly address hate", adding that the timeline would be "weeks, not months" if moderators of individual forums engage in the process.
"[Our] current policy lists only what you cannot do, articulates none of the values behind the rules, and does not explicitly take a stance on hate or racism," he said. "We will update our content policy to include a vision for Reddit and its communities to aspire to, a statement on hate, the context for the rules, and a principle that Reddit isn't to be used as a weapon."
The moves at Reddit come as social media platforms face public pressure to toughen their stance against hate speech and against calls for violence during the protests.
Last week, Twitter added a warning label in front of a tweet by Donald Trump — in which the US president used the phrase "when the looting starts, the shooting starts", in reference to the protests — saying it was "glorifying violence". Smaller rival Snapchat also said it would cease to promote Mr Trump's account on its platform because it was "inciting racial violence".
Mark Zuckerberg, Facebook chief executive, has refused to take similar action regarding the president's posts, arguing that private companies should not be an "arbiter of truth", in a move that has prompted complaints among his staff.
- FT