We demonstrate a chip-based octave-spanning soliton microcomb in a whispering gallery mode microresonator platform. By fabricating a silica microdisk resonator and optimizing its dispersion with dry etching, we achieve an octave-spanning single-soliton microcomb with a repetition rate of ∼670 GHz at an optical pump power of 162.6 mW. Also, two dispersive waves at the end of the spectrum are observed to extend the comb spectral range and improve the comb power.