How should I list an unfinished bachelor thesis in my application for a Master's programme? I am currently a final year undergraduate but I haven't finished working on my thesis yet. (I'm currently working on it nonetheless)

However a professor of mine told me to put it in the CV since it is also very related to the Master's programme.

How could I mention that it is not finished in the CV?

Writing "thesis: &lt; title >" in the CV
 without further information looks like I have at least submitted it.
I would list the title of the thesis along with an expected completion date. 

Depending on the format of your CV, this may just be the year, or, if you know the exact deadline, the date in Month, Year or DD/MM/YY format (the specifics of your long date formatting is country-dependent of course).

The listing on the CV might look something like


  Bachelors' Thesis: "My Awesome Thesis Title Goes Here." May 2017


Or


  "My Awesome Thesis Title Goes Here." (Bachelors' Thesis, expected
  completion date 30/05/17)


If you are submitting some form of covering letter as part of your application, it would also be a good idea to talk about your thesis there and also note that it is not yet complete. However, I think the admissions committee would be aware that if you are a current undergraduate student that your thesis is likely to be unfinished at this point. Good luck with your application!