MLB Commissioner Bud Selig said the league would donate $1 million to help victims of the earthquake while Armstrong, through his Livestrong foundation, has offered $250,000.
"Major League Baseball believes it is crucial to help the Haitian community in this time of need," Selig said in a statement.
"It is difficult for us to imagine the catastrophic toll this earthquake has taken on the people of Haiti, a land not far from the United States.
"We hope this contribution will help aid in the relief efforts and we encourage our fans to make a donation as well."
MLB's contribution, which will be coordinated through UNICEF, is being made on behalf of the league, its 30 teams, the MLB Network and MLB.com.
Armstrong said his foundation's donation would go to Partners in Health and Doctors Without Borders.
"Both of these groups have a strong presence in the region and have the ability to make an immediate difference in the rescue and recovery efforts that are already underway," he said on his official website (www.lancearmstrong.com).
"This nation has just suffered a devastating earthquake. It has already endured far too much tragedy and needs our help now more than ever."
More help in the sports world was coming from the NFL, NBA stars Alonzo Mourning and Dwyane Wade, and NASCAR's leading Hendrick Motorsports team.
The NFL and the players' union will contribute $1 million, split between the American Red Cross and Partners In Health.
The NFL and its network broadcast partners will also run messages before and during this weekend's playoff games to promote donations for Haiti.
In addition, NFL owners and players are making personal donations. The Washington Redskins say they are joining Diageo, one of the team's business partners, to fly medical supplies, personnel, and clothing to Haiti.
Hendrick Motorsports loaned a 45-passenger plane to Missionary Flights International, which will send support teams in and out of Port-au-Prince. The first flight is scheduled to leave Fort Pierce, Florida on Saturday and will take 30 passengers and medical supplies into Haiti.
The NASCAR team was planning to fly at least one roundtrip per day, with no timeline set on how long the plane and personnel will be on loan. Hendrick officials have a second plane on standby, and team owner Rick Hendrick was covering all costs associated with the flights.
Mourning arrived in devastated Port-au-Prince on Friday. In a statement released to The Associated Press, the retired NBA star said he and Wade were confident that they and other pro athletes "can work together to make a difference."
His former Miami Heat team also announced plans to send money and supplies, asking fans to give in exchange for ticket vouchers to games.
The Red Cross estimates 45,000 to 50,000 people were killed in Tuesday's earthquake.
There is massive devastation in Haiti from the earthquake. This is just heartwrenching. Let us come together and donate supplies and money, and keep these people who are going through unimaginable circumstances in our thoughts and prayers.