Twin　nucleation　in　a　face-centered　cubic　crystal　is　believed　to　be　accomplished　through　the　formation　of　twinning　partial　dislocations　on　consecutive　atomic　planes.　Twinning　should　thus　be　highly　unfavorable　in　face-centered　cubic　metals　with　high　twin-fault　energy　barriers,　such　as　Al,　Ni,　and　Pt,　but　instead　is　often　observed.　Here,　we　report　an　in　situ　atomic-scale　observation　of　twin　nucleation　in　nanocrystalline　Pt.　Unlike　the　classical　twinning　route,　deformation　twinning　initiated　through　the　formation　of　two　stacking　faults　separated　by　a　single　atomic　layer,　and　proceeded　with　the　emission　of　a　partial　dislocation　in　between　these　two　stacking　faults.　Through　this　route,　a　three-layer　twin　was　nucleated　without　a　mandatory　layer-by-layer　twinning　process.　This　route　is　facilitated　by　grain　boundaries,　abundant　in　nanocrystalline　metals,　that　promote　the　nucleation　of　separated　but　closely　spaced　partial　dislocations,　thus　enabling　an　effective　bypassing　of　the　high　twin-fault　energy　barrier.