We report on the first Belle search for a light CP-odd Higgs boson, A0, that decays into low mass dark matter, χ, in final states with a single photon and missing energy. We search for events produced via the dipion transition (2S)→ (1S)π+π-, followed by the on-shell process (1S)→γA0 with A0→χχ, or by the off-shell process (1S)→γχχ. Utilizing a data sample of 157.3×106 (2S) decays, we find no evidence for a signal. We set limits on the branching fractions of such processes in the mass ranges MA0<8.97 GeV/c2 and Mχ<4.44 GeV/c2. We then use the limits on the off-shell process to set competitive limits on WIMP-nucleon scattering in the WIMP mass range below 5 GeV/c2.