What happens when a roommate moves out before the lease ends?
Nothing happens to the rent. That is the whole problem. Residential leases signed by several tenants commonly make them jointly and severally liable — meaning each signer can be made to pay the entire obligation, not a personal fraction of it. Where that is how your lease and your state work, a departure doesn’t reduce the number the landlord is owed. It reduces the number of people physically present to fund it.
So the bill doesn’t shrink. It reprices. Joint and several liability lets the landlord pursue any signer for the whole amount — the departed roommate very much included. But the people still living there are the ones with the tenancy to protect, and in practice they are the ones who cover it rather than let the lease go into default.
The one-sentence version: your internal split was never the obligation. It was a private arrangement sitting on top of a single obligation, and a departure is the moment the private arrangement stops being able to carry it.
This is a guide to what actually changes at that moment, not legal advice. Landlord-tenant rules are state law and lease terms vary — the statutes below are cited as concrete illustrations of a shape that recurs, not as a description of your jurisdiction. Read your own lease and check your own state.