DraftKings and FanDuel, the two largest United States sportsbook operators, have agreed to adjust betting options to abide by MLB's desires.
The new measures by sportsbook operators cover 98% of the US betting market and are aimed at limiting integrity risks.
The move comes a day after Dominican pitchers Emmanuel Clase and Luis Ortiz of the Cleveland Guardians were indicted by federal prosecutors in New York on charges related to a scheme to rig the outcomes of specific pitches thrown in MLB games to produce major payouts on "prop" bets to wagerers tipped off about what was coming.
Both players were put on leave by MLB in July pending the outcome of its investigation into unusual betting around their pitches.
They each face up to 65 years in federal prison if convicted on all charges, which include fraud, conspiracy, and bribery.
"The risk on these pitch-level markets will be significantly mitigated by this new action targeted at the incentive to engage in misconduct," MLB said in a statement. "The creation of a strict bet limit on this type of bet, and the ban on parlaying them, reduces the payout for these markets and the ability to circumvent the new limit."
MLB commissioner Rob Manfred said since legalized sports betting began, the league has "worked with industry and regulatory stakeholders across the country to uphold our most important priority: protecting the integrity of our games for the fans.
"I also commend the industry for working with us to take action on a national solution to address the risks posed by these pitch-level markets, which are particularly vulnerable to integrity concerns."
"We look forward to continuing to work with all stakeholders, including licensed sports betting operators, elected officials and regulators, to ensure we are always protecting the integrity of our game."
Ohio governor Mike DeWine, whose in-state Guardians were at the heart of the pitch-fixing scandal, said other leagues should follow MLB's example.
"By limiting the ability to place large wagers on micro-prop bets, Major League Baseball is taking affirmative steps to protect the integrity of the game," DeWine said. "I urge other sports leagues to follow Major League Baseball's example with similar action."
MLB sportsbook partner FanDuel's president, Christian Genetski, said in a statement that the move will safeguard the integrity of baseball.
"The legalized sports betting industry is built on a foundation of dialogue and collaboration with our league partners and the states in which we operate," he said.
"This initiative illustrates our unwavering commitment to building a legal and regulated market that roots out abuses by those who seek to undermine fair competition and damage the integrity of the games we love."
