The letter, written by Rep. Alcee Hastings, D-Fla., and signed by 76 members of Congress, comes as the president faces pressure from gay rights groups to do more to address their concerns.
Chief among those concerns is the Clinton-era policy that allows gays and lesbians to serve in the military provided they do not disclose their sexual orientation. The lawmakers called the policy "misguided, unjust and flat-out discriminatory."
"Don't Ask, Don't Tell is not only an injustice to them, but a disservice to the U.S. military and our country as a whole," the letter reads.
The lawmakers urged Obama to direct the Armed Services not to initiate any investigation into the sexual orientation of service members and to disregard third-party accusations.
"Under your leadership, Congress must then repeal and replace Don't Ask, Don't Tell with a policy of inclusion and non-discrimination," the letter reads. "This bilateral strategy would allow our openly gay and lesbian service members to continue serving our country and demonstrate our nation's lasting commitment to justice and equality for all."
Though Obama pledged to end the controversial policy during his campaign, he has not taken concrete steps to do so. The lawmakers in Monday's letter said they are "confident" he will eventually keep his promise.
Obama also has not stepped in to block the dismissal of gays and lesbians who face court martial for disclosing their sexual orientation.
While he signed a memorandum last Wednesday expanding certain benefits to cover the same-sex partners of federal employees, the move did not cover the federal health insurance, retirement or survivor benefits that heterosexual couples receive.
Activists also want Obama to do more to repeal The Defense of Marriage Act -- the 1996 law which specifically prohibits extending health and retirement benefits to same-sex partners of federal employees.
Obama has said he wants to repeal it, but activists are waiting to see action.