Smells that meet certain conditions can enjoy trademark protection; however, this highly depends on a concrete jurisdiction.
In the US, it is possible to trademark a smell or an odour; however, as you can imagine, it is a bit more complicated that trademarking a logo or a brand name. Nevertheless, as long as the smell indicates the source of goods and services and is not merely functional, it can be trademarked. One of the most famous examples of such a trademark is the scent of Play-Doh produced by Hasbro.
In the EU, it is currently not possible to register a smell, also known as an olfactory mark, due to its nature, lacking stability, and durability.
In the UK, it is technically possible to register an olfactory mark; however, to this date, only two such marks have come into existence, both of which were registered in the 90's.