Michaels gave birth to a boy and girl Tuesday: son Miller Steven and daughter Johnnie Rose.
"Tammy and the babies are in excellent health," Etheridge said on her Web site. The babies were conceived using an anonymous donor from a sperm bank.
"The creation of life brings about immeasurable love, and pours hope into the future. The joy will help carry us through our upcoming sleepless nights," the couple said in a statement.
Etheridge, 45, and Michaels, 31, held a commitment ceremony in 2003. The Grammy-winning singer has two children from her relationship with former partner Julie Cypher: daughter Bailey Jean and son Beckett. Cypher had the children through artificial insemination using a sperm donation from rock legend David Crosby.
Etheridge was diagnosed with breast cancer in 2004, but is now cancer-free. She last released a greatest hits album titled "The Road Less Traveled."
Michaels starred on the 2005 NBC comedy "Committed" and has appeared on Showtime's "The L Word."