Massachusetts layers an entity-level corporate excise, a 4% surtax on high earners, and an elective pass-through excise on top of its flat income tax. Its filing dates mostly track the federal calendar, but the payment thresholds for automatic extensions are specific and easy to miss.

Massachusetts Tax Deadlines at a Glance (2026)

Massachusetts S corporations (Form 355S) and partnerships (Form 3) share the same March 16, 2026 deadline — the 15th day of the third month, rolled from Sunday, March 15 — extending to September 15. Individuals and C corporations file by April 15. There is no Patriots’ Day shift for 2026; April 15 is firm.

The 5% Rate Plus the 4% Surtax

Massachusetts taxes most income at a flat 5%. A 4% surtax applies to taxable income above an inflation-indexed threshold — $1,107,750 for 2026 — bringing the top combined rate to 9%. CPAs advising high-income clients should model the surtax on large one-time events (business sales, big capital gains) where it bites hardest.

Corporate Excise and the Elective PTE

The corporate excise is a two-part tax: 8% on net income attributable to Massachusetts, plus $2.60 per $1,000 of taxable tangible property or net worth, with a $456 minimum excise. S corporations always owe the non-income measure and minimum, and face the income-measure “sting tax” once receipts reach thresholds: 2% at $6M–$9M in receipts and 3% at $9M+. Massachusetts also offers an elective 63D PTE excise at 5% (Form 63D-ELT), giving qualified owners a refundable credit equal to 90% of their share.

Extensions, E-File, and Penalties

Automatic extensions require payment by the original date: individuals must have paid at least 80% of total tax; corporations must have paid the greater of 50% of tax or the $456 minimum. Massachusetts mandates electronic filing and payment above certain thresholds (payments of $5,000 or more). Individual estimated payments are due April 15, June 15, September 15, 2026, and January 15, 2027. Late-filing and late-payment penalties each run 1% per month (max 25%); interest was 8% for Q1 and 7% for Q2 2026.

CPA Action Steps

• File Form 355S and Form 3 by March 16; Form 355 and Form 1 by April 15

• Model the 4% surtax above $1,107,750 for 2026, especially on one-time income

• Meet the 80% (individual) or 50%/$456 (corporate) payment test to secure the extension

• Evaluate the elective 63D PTE excise for SALT-cap relief

Verify current rates and dates against the Massachusetts Department of Revenue (mass.gov/dor) before filing.