Not supported
No. Squarespace checkout only takes payments through Squarespace Payments, Stripe or PayPal (Square is for in-person sales only), and Squarespace says its Commerce API cannot be used to connect any other processor, so an Authorize.net account cannot be attached to a Squarespace store. Merchants tied to Authorize.net need a different platform, such as WooCommerce, Magento or BigCommerce.1
Biggest gotcha: The API is not a back door.
N/A — no supported checkout integration exists for this gateway and platform, so there is nothing to verify capability by capability.
A developer's first idea is often to build a small app on the Commerce API that sends Squarespace checkout payments to Authorize.net. Squarespace rules this out: its API keys article says the Commerce API cannot connect a third-party payment processor. The only processors it names are Squarespace Payments, Square (point of sale only), Stripe and PayPal. A quote that includes that integration is quoting work Squarespace does not allow.
Source: support.squarespace.com2
Squarespace lets you restyle checkout, add custom form fields and inject code, but custom code falls outside Squarespace support. Nothing on the page offers a way to add a payment method, and the checkout button and field text cannot be edited. An Accept.js form injected into the page would sit apart from Squarespace's own order and payment record, and would have no support from Squarespace.
Source: support.squarespace.com3
If you take the Authorize.net payment outside Squarespace and then push the order in with Create Order, that endpoint records the order but does not charge anyone. Under API-key authentication it is capped at 100 requests per hour per site, on top of the general 300-per-minute limit; OAuth extensions avoid the hourly cap. A busy sale day can hit that ceiling, so plan for backlog and retries.
Source: developers.squarespace.com4
For a merchant who replatforms to keep Authorize.net, Magento is a common choice, but its built-in Authorize.net integration was removed from core in 2.4.0. Guides and older stores still describe the old core settings. Plan for the official Commerce Marketplace extension from the start, and budget a migration for any store still on a pre-2.4 core setup.
Source: experienceleague.adobe.com5
Squarespace's 4242 test card only exercises Squarespace Payments or Stripe, so a trial store tells you nothing about Authorize.net behaviour. On the new platform, use a separate Authorize.net sandbox account and keep it in Live Mode. Test Mode stores nothing and returns transaction ID zero. Mixing sandbox and production keys always fails with reason code 13.
Source: developer.authorize.net6
Integration capabilities, requirements, pricing, availability, and vendor policies may change over time. UAT-Ops documents information based on the authoritative sources and testing available at the time of review.
Where newer information, testing, or vendor documentation materially changes a published claim, UAT-Ops may revise the page to reflect the most current verified information. Readers should confirm time-sensitive requirements with the relevant vendor before making production or purchasing decisions.