MLB Commissioner Bud Selig said the league would donate $1 million to help the victims of the earthquake while Armstrong, through his Livestrong foundation, has offered $250,000.
"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," Selig said in a statement.
"We hope this contribution will help aid in the relief efforts and we encourage our fans to make a donation as well."
The Haitian Red Cross said 45,000 to 50,000 people had died and three million were hurt or left homeless by Tuesday's quake that flattened buildings across entire hillsides and left many people trapped alive in the rubble.
MLB said its contribution will be coordinated through UNICEF and is being made on behalf of the league, its 30 teams, the MLB Network and MLB.com. Armstrong's donation would go to Partners in Health and Doctors Without Borders.
The NHL said it donated $100,000 on behalf of the league and its 30 teams to UNICEF's efforts to help Haiti.
"The catastrophe in Haiti requires an urgent response from the National Hockey League family," NHL Commissioner Gary Bettman said in a statement.
"The victims need food, water, temporary housing and basic medical supplies, and the emergency effort by UNICEF is helping to provide those critical commodities to communities in desperate need."
MLB's New York Yankees, winners of last year's World Series, are donating $500,000, according to a report on the team's website. (http://newyork.yankees.mlb.com)
An initial U.S. contribution of $100 million has already been pledged by President Barack Obama for Haiti relief aid.
(Writing by Mark Lamport-Stokes in Los Angeles and Frank Pingue in Toronto; Editing by Nick Mulvenney)