"It gives me great joy to announce that Heidi, Leni, Henry and I are expecting a new addition to our ever increasing family," Seal tells PEOPLE in an exclusive statement. "We all feel both excited and fortunate to be able to share this experience once again."
Klum's rep adds that the proud mama "feels great."
The German supermodel and the British singer already have a son, Henry, 9 months, and a daughter, Leni, 2, from Klum's previous relationship with Italian businessman Flavio Briatore.
"The arrival of any child brings you a lot of happiness," Seal told PEOPLE last year before the birth of their son. "If we're lucky enough, we'll have more."
Klum, 33, and Seal, 43, wed last May on a beach near the singer's home on Mexico's luxurious Costa Careyes. During their first anniversary party, which was also celebrated in Mexico, Seal crooned Nat King Cole's "Unforgettable" to Klum as 25 friends and family members looked on.
"What I love most about fatherhood," he said last year, is "the opportunity to be a part of the development process of a new life."
Last November, just two months after Henry was born, Seal told PEOPLE, "We try to spend as much time as possible together. There are days we never get out of our pajamas. We play with the kids in bed, watch TV, cook, hang out."
Added Klum: "Our kids are a constant source of amusement."
Up next for the mom-to-be: The new season of her reality show, Project Runway, is scheduled to begin July 12.
umberto Carreno / startraks photo; Dakota / Photo Image Press