Lansdale police responded to a home in the 400 block of Walnut Street, near West 5th Street, just after 11 p.m.
When officers arrived, they found 21-year-old Wesley Wayne Smith, of Philadelphia, dead on the floor of the master bedroom and a second man suffering from gunshot wounds.
According to investigators, Smith was visiting his girlfriend's apartment when the father of her children, 21-year-old Christopher Moore Jr., broke into the unit through a bathroom window and started shooting.
Smith returned fire, injuring Moore, officials said.
Moore was flown to Jefferson Hospital and is expected to survive.
An autopsy on Smith's body is scheduled for Tuesday, officials said.