The couple, who were married last May, have a 9-month-old son, Henry Guenther Ademola Dashtu Samuel. Klum also has a 2-year-old daughter, Leni, from a previous relationship with Formula One manager Flavio Briatore.
"It gives me great joy to announce that Heidi, Leni, Henry and I are expecting a new addition to our ever increasing family," Seal said in a statement Wednesday. "We all feel both excited and fortunate to be able to share this experience once again."
The 33-year-old German supermodel, host of Bravo's "Project Runway," and the Grammy-winning singer, 43, began dating in February 2004.