The present invention relates to a polysiloxane composition which contains (A) at least one type of organopolysiloxane that contains at least two unsaturated aliphatic hydrocarbon groups in the molecule, (B) at least one type of organohydrogenpolysiloxane that contains at least two silicon-bonded hydrogen atoms in the molecule, (C) a hydrosilylation reaction catalyst, and (D) inorganic particles which have a major axis to minor axis ratio of 1 to 1.5 and in which the proportion of particles having a diameter of 50 μm or greater is 5 wt. % or less of the total quantity of particles, and which satisfies prescribed conditions. The polysiloxane composition of the present invention has good processability and can provide a hard cured product having an extremely low coefficient of thermal expansion after curing.