Feline heartworm disease is a vector-borne parasitical disease caused by Dirofilaria immitis. Heartworm infection in dogs is prevalent in the Mediterranean countries. Information about the geographical distribution and epidemiological features of D. immitis infection in cats is scarce, particularly in urban stray cats that live within endemic regions for canine heartworm disease. The aim of the current study was to determine the seroprevalence of antigen and antibodies to D. immitis in feral cats in Zaragoza city, an endemic region of Spain. For this purpose, blood samples were examined for microfilariae using a direct blood smear technique and the modified Knott test. Two serological techniques for anti-D. immitis antibody detection (Solo Step® FH and in-house ELISA) and three different commercial antigen tests (DiroChek®, MegaELISA® DIRO Antigen and FASTest® HW) were performed. Blood samples from 250 stray cats were tested: 61 cats (24.40%) tested positive by the in-house ELISA, and 9 cats gave positive (3.6%) results with Solo Step® FH. The global seroprevalence of D. immitis in the feline population of the studied area of Zaragoza was 25.20% (63/250) including Solo Step® FH result and in-house ELISA. The blood exam for all samples was negative when evaluating for microfilariae and not a single cat was positive for antigen testing. This study demonstrates the presence of D. immitis infection in Zaragoza city. Veterinarians working in endemic areas should be aware of this infection in cats at risk and their susceptibility.