Fully conjugated ladder polymers (CLP) possess unique optical and electronic properties and are considered promising materials for applications in (opto)electronic devices. Poly(indenoindene) is a CLP consisting of an alternating array of five- and six-membered rings, which has remained elusive so far. Here, we report an on-surface synthesis of oligo(indenoindene) on Au(111). Its structure and a low electronic band gap have been elucidated by low-temperature scanning tunneling microscopy and spectroscopy and noncontact atomic force microscopy, complemented by density functional theory calculations. Achieving defect-free segments of oligo(indenoindene) offers exclusive insight into this CLP and provides the basis to further synthetic approaches.