Human Rights Campaign President Joe Solmonese said the United States "lost its greatest champion and strongest voice for justice, fairness and compassion. The loss to our community is immeasurable," CNN reported.
Kennedy died Tuesday of brain cancer.
"There was no greater hero for advocates of (lesbian, gay, bisexual and transgender) equality than Senator Ted Kennedy," Solmonese said in a statement.
Writer John Aravosis said Kennedy's support for gay rights was instrumental in advancing the issue.
"On gay rights, he was absurdly helpful for a straight senator back before gay rights were popular," said Aravosis, who runs AmericaBlog, a political site that frequently tackles gay rights issues.
Former Massachusetts state Sen. Jarrett Barrios, incoming president of the Gay and Lesbian Alliance Against Defamation, also praised Kennedy's work.
"In those early years, his support may have turned heads but (that) didn't dampen his support -- and eventually helped change hearts and minds about LGBT equality in the Senate and around the country," Barrios said in a statement.