Melem and its derivatives were synthesized from melamine by a two-step heat treatment method between 400 and 650 °C in air. It was demonstrated that such polymeric C–N semiconductors possess a photoelectrochemical (PEC) conversion effect with both photocathodic and photoanodic characteristics, which was proposed to be caused by the formation of tri-s-triazine ring. The dimelem synthesized at 450 °C exhibited the highest PEC conversion activity. The possible reasons were discussed. The unique bidirectional photocurrent generation makes melem and its derivatives attractive photoelectrochemical materials.