Only some are, and then only the newer versions.
EF 16-35mm f/2.8L USM
EF 24-70mm f/2.8L USM
EF 28-300mm f/3.5-5.6L IS USM
EF 70-200mm f/2.8L IS USM
EF 300mm f/2.8L IS USM
EF 400mm f/2.8L IS USM
EF 400mm f/4 DO IS USM
EF 500mm f/4L IS USM
EF 600mm f/4L IS USM
Extender EF 1.4x II
Extender EF 2x II
EF 16-35mm f/2.8L II USM
That's the official Canon list, other lenses like the 70-200 f/4IS have dustproofing but not full-on weathersealing of all parts.