Squarespace offers an article explaining how to move a WordPress site to SquareSpace.
This can move content and domain. Content includes attachments, blog pages, authors, categories, comments, and MOST images.
The instructions from SquareSpace explain that a site's layout, design and fonts cannot move. It may be easier to start from a new template and focus on moving content into it.
Also be sure to read the instructions on what kinds of images don't get imported.
SquareSpace has a different ecosystem of plugins from WordPress, so any plugins that the WordPress site relied upon will need to be considered separately.
The source code (markdown) for this article can be found on GitHub.