For SEO, default to a subfolder for your blog, because it keeps the content inside your main domain’s authority and topical signals rather than splitting them off. A subfolder like yoursite.com/blog is treated as part of the main site, so the authority and relevance the blog earns flow into the domain and the domain’s strength flows into the blog. A subdomain like blog.yoursite.com can be treated more as a separate property, which means it may not share authority as fully in either direction. Choose the subfolder unless a strong technical or organizational reason forces the subdomain.
The reasoning is consolidation of signals. When everything lives under one domain in subfolders, links earned by your blog, the topical depth it builds, and the site’s overall authority reinforce each other within a single entity. Splitting the blog onto a subdomain risks fragmenting that, since the blog and the main site can end up building authority somewhat independently rather than compounding. Keeping the blog within the domain is safer precisely because it avoids that fragmentation. This consolidation reasoning reflects how search engines have generally been observed to treat the two structures, and it is worth confirming against current guidance since the exact handling is debated.
That said, neither extreme is right. It is not true that subfolders and subdomains are identical for SEO, the consolidation difference is real enough to make the subfolder the sensible default. It is also not true that you should never use a subdomain. Real reasons exist: a separate technical stack or CMS the main site cannot host, a genuinely distinct brand or audience, security or organizational boundaries, or platform constraints that make a subfolder impractical. When one of those forces the issue, a subdomain is a reasonable call, and the SEO cost can be managed with strong internal linking and consistent branding.
For your blog, plan it as a subfolder of your main domain unless you hit a concrete technical or organizational wall that genuinely requires a subdomain. Make the subfolder the default you have to argue your way out of, not the option you have to argue your way into, and your blog’s authority and your domain’s authority will keep building each other.