Games between the Cincinnati Reds and Brewers in Milwaukee, Seattle Mariners and Padres in San Diego and the Los Angeles Dodgers and Giants in San Francisco were called off hours before they were set to begin.
Dodgers star Mookie Betts, who is Black, told his teammates he was sitting out and they backed him.
"For me, I think no matter what, I wasn't going to play tonight," Betts said.
"I have to use my platform to at least get the ball rolling,'' he said.
All three postponed games will be made up as doubleheaders Thursday. There is the possibility, too, that other games around the majors could be affected on Thursday -- one day before MLB is set to celebrate Jackie Robinson Day, which was postponed from April by the coronavirus pandemic.
Once Betts made his decision, the Dodgers stood by him, ace Clayton Kershaw said.
"More than anything as a teammate of Mookie's, as a member of this team ... as a white player on this team is how do we show support? What's something tangible that we can do to help our Black brothers on this team?'' Kershaw said.
Other MLB games had finished, were in progress or just about to start as the announcements were made.
The baseball postponements came after the NBA's Milwaukee Bucks didn't come out onto the floor for Game 5 of their first-round playoff series with the Orlando Magic on Wednesday afternoon at Lake Buena Vista, Florida. NBA officials later announced that all three of the day's scheduled playoff games had been postponed.
Three WNBA games also were called off inside the league's bubble in Bradenton, Florida.
MLB players drove the decisions that resulted in the postponements. Others, such as Colorado outfielder Matt Kemp, opted to sit out while their teams played.
"Given the pain in the communities of Wisconsin and beyond following the shooting of Jacob Blake, we respect the decisions of a number of players not to play tonight. Major League Baseball remains united for change in our society and we will be allies in the fight to end racism and injustice," the league said in a statement.
Said players' union head Tony Clark: "At this critical time, players have been deeply affected by the recent events in Wisconsin and by similar events in other parts of the country. We are proud of the stand that our players have taken, and we remain committed to supporting their efforts to effect change in MLB communities and beyond.''
Blake was shot seven times by police on Sunday. He was shot as he attempted to enter the driver's side door of his vehicle with three of his children inside. Video of the shooting was distributed on social media.
"The players from the Brewers and Reds have decided to not play tonight's baseball game. With our community and our nation in such pain, we wanted to draw as much attention to the issues that really matter, especially racial injustice and systemic oppression," Milwaukee and Cincinnati players said in a joint statement at Miller Park.
Said Brewers manager Craig Counsell: "The Bucks led here. The NBA led here. But our players, they went first in Major League Baseball and I'm still very proud of them for that."
The Brewers supported that stance.
"The Milwaukee Brewers organization joins the players in their decision to not play tonight's game. We need to pause and reflect on the events that are causing such pain and hardship to our local community and country. The entire organization is committed to putting the spotlight on racial injustice, inequality, and the necessity for change," the team said.
Milwaukee's Ryan Braun said the Bucks' decision inspired the Brewers to act. The team held a meeting Wednesday afternoon, which led to the decision not to play.
"That motivated us. ... At some point, actions speak louder than words," Braun said in a video news conference. "Because this happened so close to home, it hits us differently than it does other teams."
Braun said the team's intention is to continue with the season as scheduled, but "this was our top priority today."
Brewers star Christian Yelich said he exchanged texts with Bucks guard Pat Connaughton to let him know they wouldn't be playing at Miller Park and that "we wanted to be united with them in what they started.''
Brewers player representative Brent Suter said he informed Cincinnati's Mike Moustakas and pitcher Wade Miley -- both former Brewers -- of the decision.
"They just said flat out, 'We support you guys no matter what. Whatever you decide to do, we're all in favor. We want to follow your lead,'" Suter said. "So that was a great comfort for us going to the meeting.''
Mariners infielder/outfielder Dee Gordon said in a tweet that the team decided unanimously to skip the game at Petco Park.
The Mariners have the most Black players of any team in MLB, and pitcher Justin Dunn tweeted: "Enough is enough."
Dunn included a Black Lives Matter hashtag and a cartoon image of he and his Black teammates in Black Lives Matter shirts.
A sprinkling of players from the Dodgers and Giants were loosening up and the grounds crew was prepping the field at Oracle Park when the game was postponed.
"My cousin got shot and killed. My father was one of the first black men in his high school,'' said Dodgers manager Dave Roberts, son of an African American father and Japanese mother. "It was just more of getting everyone together and sharing our thoughts. ... Collectively, ultimately we came to the consensus that we shouldn't play today.''
Kemp, who is Black, announced on social media he would skip Colorado's game in Arizona "in protest of the injustices my people continue to suffer."
"I could not play this game I love so much tonight knowing the hurt and anguish my people continue to feel," he wrote. "In a world where we are the ones who need to remain calm while a trained professional points a gun in our face; a world where the people in uniforms who took an oath to protect us are the same ones killing us; a world where we become hashtags before we even reach our potential; we must stand together, speak out, protest, and be the change we demand, require, and need so bad."
Mets slugger Dom Smith, a Black man who has spoken about his experiences in a predominantly white sport, took a knee for the national anthem for the first time this season. New York pitcher Robert Gsellman, who is white, wore a Black Lives Matter T-shirt instead of a uniform as he watched from the stands behind the dugout.
"Ending police brutality is more important than sports," Pirates infielder Cole Tuckerwrote in a tweet. Pittsburgh played the Chicago White Sox in the afternoon, before the Bucks became the first pro sports team to decide it would not play Wednesday.
Dexter Fowler and Jack Flaherty of the Cardinals, who are Black, both opted not to participate in St. Louis' game against visiting Kansas City. The team tweeted that it supported the decision.
Chicago Cubs outfielder Jason Heyward, who is Black, was removed from the lineup shortly before first pitch in Detroit.
"There were multiple guys saying they weren't comfortable going out there and playing if I wasn't going to go out there. They didn't want to leave me hanging," Heyward said. "I let them know, encouraged them -- no, go play the game. I don't think the game should be canceled. But I think I have to do what I have to do.''
Washington manager Dave Martinez said he hadn't heard about the movement before a 3-2 loss to Philadelphia.
"I just now started reading what was happening and what was going on. I will say this, though. I'm proud of the NBA. I'm proud of all the people who stand for justice,'' he said. "It's horrible. We need change.''
"I'm going to talk to the players. We'll see what happens tomorrow. I get it," he said, choking up.
Toronto slugger Rowdy Tellez said the Blue Jays would meet Thursday to decide a course of action.
"It's going to be a great team discussion and probably a very emotional one for a lot of guys that know what it's like," he said.
The Associated Press contributed to this report.
Brewers, Reds call off game following Bucks' boycott
Jeff Passan says Major League Baseball was surprised by the Brewers' decision to call off their game against the Reds after the Bucks decided to boycott their NBA playoff game.