(2024) Sorbonne (and normally CNRS too) reimburses 75% of your transportation fees. If you are not sure wether you have it or not, check on your paycheck and you should have a line like “REMBT DOMICILE-TRAVAIL” (refund home-work). Normally Malika automatically asks for it when creating the contract, but if not you should go ask her.
Sustainable mobility : If you regularly come to work with a “sustainable mbility” -bike, carpooling, scooter, etc… - you can ask for a bonus. For this you have to go on the sorbonne intranet : https://intranet.sorbonne-universite.fr/fr/ressources-humaines/remuneration-1/prise-en-charge-frais-de-transport/forfait-mobilites-durables-2.html (in French sorry :/ ) et send back to HR before December 31 each year the file given there, filling it with the number of days on which you used these transports throughout the year. Depending on this number, you can get a bonus up to 300€ ! It is cumulative with the reimbursement of your Navigo.
(2022) For PhD Student : If your age is under 26 years old you're eligible to an ImagineR transport card. it's a one-year “cheap” contract that you can do as student (you will need your “attestation d'inscription”, un RIB, etc.). You will pay only about 35 euro/month and SU reimburse 50% (described above). For more details https://www.iledefrance-mobilites.fr/titres-et-tarifs/detail/forfait-imagine-r-scolaire .. (Wafa)
(2021) For everyone : To apply for Navigo Pass Annuelle, and pay per month (It’s 75.20 Euro/month as of 20210924). For subscription, you will need to pay an extra 7.60 Euros for the first time. First make sure you have all these:
General rule of thumb to get things move fast, always be ready with all the documents. Print several copies of them and give it in whenever anything seems to be in doubt There are two ways you can do it. If you have everything at the earlier half of the month prior to the start of the pass (e.g. You are ready by 15th of September to apply for a pass starting in October), then you can choose to do it online. Pros, it's quite straightforward, you can choose to read in English; Cons, you might need to wait to get your card.
In-person Option Go to a train/metro station that provides the service, so far in my knowledge (my == Mafer), you can do that in Jussieu/Maison Blanche for weekdays, but on weekend Porte de Choisy is better. Bring along all the documents and the personnel will do it for you. Pros, you get your card straightaway, if you miss out anything they will tell you straight; Cons, if you can't speak French well, you need some luck to meet a nice staff (I met one!). You should be able to get your card on the spot.
(Mafer, with small edits by Sharbat)