The most serious charge was filed against Derek Chauvin, whose caught-on-video treatment of the handcuffed Floyd spurred worldwide protests.
Chauvin pressed his knee into Floyd’s neck for nearly nine minutes, despite Floyd saying he couldn’t breathe.
Three other officers – Thomas Lane, J Kueng and Tou Thao – were charged with aiding and abetting second-degree murder.
All four were fired last week.
The new charges were filed by Minnesota Attorney General Keith Ellison, who planned an announcement later on Wednesday.
Lawyer for Floyd’s family, Benjamin Crump, called it “a bittersweet moment” and “a significant step forward on the road to justice”.
Crump said Ellison had told the family he would continue his investigation into Floyd’s death and upgrade the charge to first-degree murder if warranted.
Earlier on Wednesday, in a visit to a makeshift shrine at the street corner where Floyd died, his family had again called for the arrests of Lane, Kueng and Thao.
Minnesota has opened a civil rights investigation into whether the Minneapolis Police Department has a pattern of discrimination against minorities.
US protesters have ignored curfews as they vent anger over Floyd’s death, but street violence appears to have subsided.
Meanwhile US President Donald Trump’s defence secretary says he is opposed to sending in troops, after the president threatened to call in the military to crack down on demonstrators.
Tens of thousands of people took to the streets of US cities coast to coast for an eighth night in protest over Floyd’s death and brutality against other black Americans.
Authorities took the unusual step of ordering curfews, and bands of police in riot gear and other heavily armed officers patrolled, ringing landmarks and shouting at protesters while helicopters roared overhead.
While most protests have been peaceful, there appeared to be less looting and vandalism, and clashes between police and protesters were more sporadic.
Defense Secretary Mark Esper said he did not back deploying troops to patrol the country.
“The option to use active duty forces in a law enforcement role should only be used as a matter of last resort and only in the most urgent and dire of situations. We are not in one of those situations now,” he told a news briefing on Wednesday.
Trump had threatened to use the military to battle violence and had derided local authorities, including state governors, for a response he said failed to dominate the disturbances.
In New York, which last ordered a curfew 75 years earlier during World War II, Mayor Bill de Blasio said an 8pm curfew would be in place until Monday.
Clashes between protesters and police and looting of some stores in New York gave way to relative quiet in the early hours.
Police made 200 arrests, largely for curfew violations.
In Los Angeles, many demonstrators who defied a curfew were arrested, but calm was restored by mid-evening to the extent that television switched back to regular programming.
Large marches and rallies also took place in Philadelphia, Atlanta, Denver and Seattle, and smaller ones in many other communities.
Massive crowds have gathered internationally, from Paris to Nairobi.
In London, tens of thousands chanted “no justice, no peace, no racist police” and “Black lives matter” in a march on Wednesday.
On Wednesday, Trump defended his Monday night walk to pose with a Bible in front of a church, after peaceful protesters had been tear-gassed and been fired upon with rubber bullets to clear a path from the White House.
Washington police punched and clubbed an Australian media crew at the protest.
“Most religious leaders loved it. It’s only the other side that didn’t like it, you know, the opposing, the opposition party,” Trump told Fox News.
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