Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel production. In this study, a novel heterostructure formed between a conjugated polymer poly-2,6-diaminopyridine (PDAP) and three-dimensional TiO2 microspheres was grown in situ on a Ti substrate (PDAP-3DTiO2MSs/Ti) and used as photoanode for water oxidation in alkaline media under AM 1.5G illumination. The PDAP-3DTiO2MSs/Ti can produce applied bias photon-to-current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm−2 at 1.23 V vs. RHE. Moreover, PDAP-3DTiO2MSs/Ti displays impressive photoelectrochemical stability with 93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical characterization and photo-/electro-chemical measurements, the superior PEC water splitting performance of PDAP-3DTiO2MSs/Ti should benefit from the coexistence of Ti3+ and Ti4+ in 3DTiO2MSs, the light harvest capability of PDAP and the type II heterojunction formed between 3DTiO2MSs and PDAP, which result in the enhanced generation and separation of photocarriers. Display omitted •A novel heterostructure formed between PDAP and 3DTiO2 microspheres.•The photoanode achieved a high applied bias photon-to-current efficiency at low bias.•A photocurrent density of 1.56 mA cm−2 at 1.23 V vs. RHE under AM 1.5G.•Type II heterojunction and coexistence of Ti3+ and Ti4+ improved the PEC performance.