Baldwin, 50, an attorney and seven-term Democratic congresswoman, defeated four-term former Wisconsin Gov. Tommy Thompson in the statewide race that also put Wisconsin in President Obama's win column despite native son Rep. Paul Ryan being the Republican vice presidential nominee.
Ryan was re-elected to his U.S. House seat. Baldwin was elected to the seat of retiring four-term Democratic Sen. Herb Kohl.
Baldwin won 51 percent of the vote in a state that decisively banned gay marriage in a constitutional amendment six years ago, CNN reported.
"The people's voice was heard tonight, Wisconsin, and come January your voice will be heard in the United States Senate," Baldwin told cheering supporters in Madison. "I didn't run to make history, I ran to make a difference," she said.
She was elected to represent Wisconsin's second congressional district in 1998 and led efforts that passed hate crimes legislation and legislation to extend benefits to same-sex partners of federal employees.
"For the LGBT [lesbian, gay, bisexual, and transgender] person growing up in Wisconsin or anywhere across [the] country, seeing an openly gay woman who is able to rise up to become a senator in the U.S. Congress is an incredible role model," Chad Griffin, president of the gay-rights group Human Rights Commission, said in a statement.
The senate race was one of most expensive in Wisconsin history with nearly $66 million spent, $46 million of that by outside groups," the Milwaukee Journal Sentinel said.