This　work　considers　the　physical-layer　security　of　a　wiretap　channel,　where　a　transmitter　communicates　with　a　receiver　using　the　precoding-aided　spatial　modulation　(PSM)　in　the　presence　of　an　unauthorized　passive　eavesdropper.　We　first　analyze　the　security　capability　of　the　PSM　and　show　that　the　PSM　has　the　property　of　low-probability-of-interception　(LPI).　Based　on　the　observations,　we　then　design　a　secret　PSM　(SPSM),　which　represents　a　generalization　of　the　PSM.　We　derive　the　upperbounds　for　the　bit　error　rate　(BER)　of　both　the　desired　receiver　and　the　eavesdropper　when　assuming　communications　over　Rayleigh　fading　channels.　Both　the　numerical　results　evaluated　from　the　BER　upper-bounds　and　the　simulation　results　are　provided　to　demonstrate　the　secrecy　performance　of　the　SPSM.　Our　studies　show　that　the　SPSM　can　significantly　enhance　the　security　of　the　PSM,　in　addition　to　inheriting　all　its　merits.　©　2015　IEEE.