Lampson, who is seeking the Democratic nomination for Congress, made his announcement at a restaurant before about 100 supporters. He filed the paperwork Friday in Austin.
DeLay, a Republican, is seeking a 12th term in his suburban Houston district.
Lampson, a conservative Democrat, represented an adjacent district for eight years until redistricting that DeLay engineered cost him enough Democratic votes that he lost to U.S. Rep. Ted Poe, R-Houston, in 2004.
Lampson is the first Democrat with name recognition and political experience to challenge DeLay since he was first elected in 1984.
DeLay had to step down as majority leader in September after a Travis County grand jury indicted him in a campaign finance investigation. He has denied wrongdoing.