The group, led by Sen. Kirsten Gillibrand, D-N.Y, wants Medvedev to intervene with Russian authorities to ensure that adoptions of Russian children by American families, especially those already being processed, not be halted.
Russian authorities have said all adoptions to the U.S. are on hold pending a new agreement on the matter that could take months.
In a letter to be sent on Friday to Medvedev and Secretary of State Hillary Rodham Clinton, Gillibrand, four other senators and 12 House members say they understand the concerns that led Russia to impose the freeze.
Moscow acted following an incident this month in which an American woman put her adopted Russian son, 8-year-old Artyom Savelyev, on a plane alone to Moscow. She said she "no longer wishes to parent" the boy.
A U.S. delegation is to travel to Moscow next week to discuss a new adoption agreement.
The lawmakers appealed to Medvedev, urging him to ensure that "adoptions between our two countries — particularly those already in process — will continue."
They said they "stand ready to support such improvements as are needed to serve the best interests of the children and their new families."
Russia has not formally informed Washington of the suspension in adoptions, according to U.S. officials. But officials say at least some adoptions to the U.S. have been frozen as a result of the incident.
More than 1,800 children from Russia were adopted in the United States last year, according to the Russian Education and Science Ministry.