The transcription factor NF-κB has been associated with a range of pathological conditions of the heart, mainly based on its function as a master regulator of inflammation and pro-survival factor. Here, we addressed the question what effects activation of NF-κB can have during murine heart development. We expressed a constitutively active (CA) mutant of IKK2, the kinase activating canonical NF-κB signaling, specifically in cardiomyocytes under the control of the α-myosin heavy chain promoter. Expression of IKK2-CA resulted in embryonic lethality around E13. Embryos showed defects in compact zone formation and the contractile apparatus, and overall were characterized by widespread inflammation with infiltration of myeloid cells. Gene expression analysis suggested an interferon type I signature, with increased expression of interferon regulatory factors. While apoptosis of cardiomyocytes was only increased at later stages, their proliferation was decreased early on, providing an explanation for the disturbed compact zone formation. Mechanistically, this could be explained by activation of the JAK/STAT axis and increased expression of the cell cycle inhibitor p21. A rescue experiment with an IκBα superrepressor demonstrated that the phenotype was dependent on NF-κB. We conclude that activation of NF-κB is detrimental during normal heart development due to excessive activation of pro-inflammatory pathways.