Calendar-year S corporations and partnerships that took a six-month extension on their 2025 returns face a September 15 filing deadline, as tax filing guidance for Form 1120-S and Form 1065 filers lays out. That is tomorrow, and unlike an individual return there is no second extension waiting behind it.
This catches more self-employed people than it should. Plenty of one-person businesses elected S corporation status to reduce self-employment tax, then discovered the entity carries a filing calendar of its own that has nothing to do with the April deadline they were used to.
What Is Actually Due On September 15
Calendar-year S corporations file Form 1120-S, originally due March 15, 2026. Filing Form 7004 pushed that six months out to September 15.
Calendar-year partnerships and multi-member LLCs taxed as partnerships file Form 1065, which was due March 16 this year because the usual date fell on a weekend. The same six-month extension moves that return to September 15 as well.
Neither return is where the tax gets paid. Both are pass-through entities, so the return’s real job is generating the Schedule K-1 that tells each owner what to report on a personal return, which is why a late filing creates a problem for every owner rather than just the business.
Why This Matters For Self-Employed Owners
The penalty structure is what makes this deadline expensive. Late filing runs $255 for each partner or shareholder for each month or part of a month the return is late, and the charge can run as long as 12 months.
Read that per-owner clause carefully. A two-person partnership that files three weeks late is not a few weeks late on one return, it is two owners times one month, because any part of a month counts as a full month.
There is a second cost that does not show up as a penalty. Owners cannot finish their own returns without a K-1, so a late entity filing pushes the delay onto every individual behind it, and anyone waiting on that document for a mortgage application or a loan file feels it immediately.
What Self-Employed Readers Should Do Next
Confirm today which entity type you actually file as. Single-member LLCs that never made an S election report on Schedule C with the personal return and have nothing due tomorrow, while an S election made in a prior year means Form 1120-S is due whether or not the business earned anything.
If the return is not going to be finished, file it anyway rather than waiting for perfection. A filed return stops the per-owner monthly clock, and an amended return later costs far less than 12 months of penalties on every owner.
Then look at your Q3 estimated payment, which shares the same date. Our coverage of the third-quarter estimated tax deadline walks through how to size that payment when income has moved during the year.
What To Watch Next
If you miss the date and the cause was genuinely outside your control, penalty relief is worth pursuing rather than assuming the charge is final. The IRS has been reworking its penalty abatement process for 2027, so the request path may look different next year than it does now.
For anyone weighing an S election for 2027, treat this deadline as part of the cost. The self-employment tax savings are real, but so is a separate return, a payroll obligation, and a filing calendar that a sole proprietor does not carry.
