Surface nitrogen structures on Fe(100) obtained by bulk-to-surface nitrogen segregation are studied using a combination of high-resolution X-ray photoelectron spectroscopy (XPS) and real-space imaging using scanning tunneling microscopy (STM). The core-level XPS N 1s results show one sharp peak, suggesting that the N atoms are mainly located in one site. The binding energy is consistent with the literature value of the Fe(100)/c(2 × 2)-N structure, for which the nitrogen atoms reside in four-fold hollow sites. Furthermore, the STM-images show regions of well-ordered Fe(100)/c(2 × 2)-N structure and regions with a high density of anti-phase domain boundaries. Regions with narrow stripe-like, 3 N atoms wide, anti-phase c(2 × 2)-N domains were observed. The anti-phase domain boundaries between the stripe-shaped domains have higher N coverage than within large well-ordered Fe(100)/c(2 × 2)-N domains.