Guam's Tax System is already described in detail elsewhere on this page - as a mirror-code US territory, Guam's assessment mechanism live-mirrors current US federal law (see Corporate/Personal Tax Rate sections above).
Guam follows the US federal tax year (calendar year) given its mirror-code status, with filing deadlines tracking current US federal deadlines live.
Guam is one of three "mirror Code" possessions (with Guam/USVI/CNMI all mirroring each other) - the Internal Revenue Code of 1986, as amended, serves AS Guam's own local corporate tax law, substituting "Guam" for "United States" wherever the Code refers to the US. This means Guam's corporate rate tracks the current US federal corporate rate live: 21% (the flat federal rate since the 2017 Tax Cuts and Jobs Act), not a rate frozen at an earlier point in time. Guam's local government has at times offered rebate programs that reduce effective tax liability below the mirrored federal rate for qualifying taxpayers - confirm current rebate program status directly with Guam's Department of Revenue and Taxation.
For the same mirror-code reason, Guam's personal income tax uses the current US federal individual brackets directly, topping out at 37%. A bona fide resident of Guam generally files and pays tax only with Guam's own local tax authority rather than the IRS, and does not owe US federal income tax on Guam-source income.
Guam has no conventional VAT or retail sales tax. Instead, a Business Privilege Tax (BPT) of 5% applies broadly to gross receipts from business activity conducted in Guam - a gross-receipts tax rather than a value-added or point-of-sale consumption tax, meaning it can cascade through a supply chain differently than a true VAT would.
Guam follows the federal "bona fide resident" test used across the mirror-code and non-mirror-code US territories alike: broadly, presence for at least 183 days in the tax year, no closer connection to the US or a foreign country, and a tax home in Guam for the full year, per IRC Section 937 and its implementing regulations. Bona fide residents are taxed on worldwide income by Guam itself (their sole filing obligation in most cases); non-bona-fide residents are taxed only on Guam-source and US-effectively-connected income.
A non-US entity has a Guam permanent establishment on the same basis as under the mirrored US Internal Revenue Code (see 26 U.S.C. Section 931 already cited elsewhere on this page).
Because Guam mirrors the full US Internal Revenue Code as its own local law, it also mirrors the US's own Subpart F and GILTI (Net CFC Tested Income, NCTI, post-OBBBA) CFC regimes in their entirety - a Guam-incorporated or Guam-resident-controlled foreign corporation is subject to the same CFC attribution rules the mainland US applies to its own residents.
As a mirror-code jurisdiction, Guam applies the same US federal interest-limitation rules as the mainland - IRC Section 163(j)'s 30%-of-adjusted-taxable-income cap on net business interest expense - rather than a separately-enacted local thin capitalization ratio.
As already described elsewhere on this page, Guam mirrors the full US Internal Revenue Code including its Subpart F/GILTI-NCTI CFC framework; the US's own hybrid-mismatch-adjacent provisions apply identically given the live mirror-code mechanism.
No foreign bank account or foreign financial asset reporting regime exists requiring residents to separately disclose foreign accounts; foreign income is reported through the standard annual tax return.
As already described elsewhere on this page, Guam mirrors the US's own IRC Section 245A 100% dividends-received deduction for qualifying foreign-source dividends given its live mirror-code status.
As already described elsewhere on this page, Guam mirrors the US's own IRC Section 901 foreign tax credit regime (Forms 1116/1118) live given its mirror-code status.
Guam does not independently negotiate its own tax treaties as a US territory rather than a sovereign state; its residents' treaty position depends on the interaction between US treaty law and the territory's own mirror-code status, which is a genuinely technical area - confirm treaty applicability directly with a US international tax adviser for any Guam-specific cross-border structure rather than assuming mainland US treaty benefits apply automatically.