C-Corp Extended Deadline — October 15
Thursday, October 15, 2026
Final extended deadline for C-Corporation Form 1120 — October 15, 2026.
Who must file or pay
C-Corporations that filed Form 7004 by April 15 for a 6-month extension.
What you need to know
C-Corporations that extended their April 15 deadline have until October 15, 2026 to file. No further extensions are granted after this date.
For startups preparing for fundraising rounds, having a clean, filed C-Corp return is a standard due diligence item. Many VCs and strategic acquirers will request the last 3 years of corporate returns in their diligence process — timely filing is a hygiene requirement.
R&D tax credits (IRC §41) must be claimed on the return when filed — they cannot be claimed retroactively without amending. Make sure your 2025 R&D credit analysis is complete before the October 15 filing.
Unpaid tax continues to accrue failure-to-pay penalties and interest from the original April 15 deadline. No further extensions beyond October 15.
Required forms
- →Form 1120
- →Form 7004 (already filed)
A licensed TaxosAgent professional reviews your situation before this deadline — so you maximize savings, not just compliance.
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Other dates to know
Individual Extension Deadline — October 15
Final extended deadline to file your 2025 individual federal tax return — October 15, 2026.
S-Corp Return Deadline — March 17
S-Corporation Form 1120-S due March 17, 2026 — shareholder K-1s must be issued by this date.
Partnership Return Deadline — March 17
Partnership Form 1065 due March 17, 2026 — K-1s to all partners by this date.