Shopify
Three routes, depending on whether the reviews already exist somewhere else, live in your store, or do not exist yet.
How to add reviews to Squarespace
The exact steps for Squarespace, in about five minutes, without a developer.
The short answer
Build a widget, copy one line of code, and paste it into a Code block. Edit the page → add a block. No developer and no theme files.
Step by step
Build the widget and connect your review sources
Choose a layout and match the colours to your site
Copy the embed code
In Squarespace: Edit the page → add a block
Add a Code block and paste the code
Save and check the live page
Where to put them
A Code block where the reviews belong, or Settings → Advanced → Code Injection for site-wide placement.
Full-width sections suit carousel and grid layouts; narrow columns suit a badge.
Does Squarespace have reviews built in?
On commerce plans Squarespace has product reviews for things you sell. They do not cover reviews left anywhere outside the store.
If your reviews already live on Google or a marketplace, nothing built into a site builder will reach them.
If they do not appear
Code blocks do not render in the editor preview on some templates. Save and view the live page before assuming it is broken.
The Squarespace troubleshooting page works through the rest in order.
Questions
Other setup guides
Any website
Whether your reviews already exist somewhere else, live in your store, or do not exist yet — three different answers.
WordPress
The exact steps for WordPress, in about five minutes, without a developer.
Wix
The exact steps for Wix, in about five minutes, without a developer.