Multibeam　antennas　have　found　more　and　more　applications　in　current　and　future　communication　systems.　In　this　letter,　a　low-profile　multibeam　fully　planar　folded　transmitarray　antenna　(FTA)　based　on　the　bifocal　principle　is　proposed.　It　consists　of　a　reflectarray,　a　transmitarray,　and　a　group　of　identical　planar　feeding　antennas.　The　transmitarray　is　developed　with　the　metasurface　and　achieves　high　transmission　efficiency　and　3　b　compensation　phases.　A　new　technique　is　proposed　to　improve　the　isolations　among　the　feeding　antennas　by　taking　advantage　of　the　multiplicity　of　the　feeding　elements.　As　an　example,　a　multibeam　FTA　operating　at　the　Ku-band　was　designed,　fabricated,　and　tested.　Experimental　results　achieve　a　peak　gain　of　22.7　dBi　and　a　scanning　range　of　+/-　21　degrees,　with　a　maximum　feed　isolation　of　24　dB.