The short answer
There's no general federal law banning smoking in private, market-rate apartments, so your landlord isn't automatically required to stop a neighbor from smoking. But that doesn't mean you have no options: your lease, your city or state's laws, and your basic right to quiet enjoyment of your home can all come into play depending on where you live and what your lease says.
The one federal rule that does exist — and its real scope
Federal law does require one specific category of housing to go smoke-free: public housing operated by a local housing authority. Under a HUD rule, those housing authorities must prohibit smoking inside every public housing unit and common area, and within 25 feet of the buildings (24 C.F.R. § 965.653). That rule is a useful data point, but it's easy to over-read. HUD was explicit when it adopted the rule that it does not apply to tribal housing, mixed-finance developments, or converted project-based assistance properties, and that outside public housing, HUD only "continue[s] to promote voluntary adoption of smoke-free policies" rather than requiring it (81 Fed. Reg. 87430). If you rent from a private landlord, this federal rule almost certainly doesn't apply to your building directly.
What your lease already says
Check your lease first. Many leases in multi-unit buildings already prohibit smoking inside units or in certain common areas, especially in buildings built or renovated more recently. If your lease has a no-smoking clause and a neighbor is violating it, that's a straightforward lease violation you can report to your landlord in writing, and it puts the responsibility for enforcement squarely on them.
Local and state smoke-free laws
A number of cities and some states have their own laws restricting smoking in multi-unit housing, sometimes covering units and sometimes just common areas and building entrances. These vary a great deal by location and change over time, so check your specific city and state government sites rather than assuming a rule applies. If your building is covered by a local smoke-free housing law your landlord isn't enforcing, a written complaint citing that ordinance carries more weight than a general request.
When you're searching for your city or county's rule, look for terms like "smoke-free housing ordinance" or "multi-unit smoking restrictions" on the local government's own site rather than a general web search, since secondhand results are often outdated or describe a different city's law entirely. Some of these ordinances only cover buildings above a certain number of units, or only newly constructed buildings, so read the actual scope before assuming your building is covered.
When smoke becomes a habitability or quiet-enjoyment problem
Even without a smoking ban, persistent smoke that seeps into your unit through shared vents, walls, or plumbing chases can rise to the level of a habitability issue if it's severe enough to affect your health or make the unit genuinely unlivable, or a breach of your right to quiet enjoyment if it's frequent and disruptive. This is a harder case to make than a straightforward lease violation, and how far it gets depends heavily on your state's law and how documented the problem is.
Building your case
Keep a log with dates, times, and how the smoke affected you: the smell, any symptoms, having to leave a room or open windows in cold weather. Photograph any visible smoke staining if it exists. Send your complaint to your landlord in writing, ask what they intend to do, and follow up in writing again if nothing changes. If the smoking is happening in a shared space where it's already banned by your lease or local law, that written record is what supports asking your landlord to enforce it, or escalating to your city's code enforcement office if they won't.
If the source is a specific neighboring unit rather than a common area, your landlord may be limited in what they can do beyond asking that tenant to stop, especially if nothing in that tenant's own lease prohibits it — which is one more reason to check your own lease's language early, since it tells you what leverage actually exists before you're relying on your landlord's goodwill alone.
Honestkey™ can review your lease for smoking-related clauses so you know exactly what you can hold your landlord to before you complain.