Well, that depends on how you define 'part of a series' and what you mean by 'Alpha' because the naming here is famously confusing. The main book is 'Alpha' by Jasinda Wilder. It's a standalone novel about a character named Ian and Thea—technically, there's no series with the title 'Alpha Ian and Thea.' However, Jasinda Wilder wrote a whole bunch of interconnected books set in the same world, like 'Beta' and 'Omega,' but those follow different couples. Some characters from 'Alpha' might pop up, but Ian and Thea's story is contained in that one book. Honestly, the marketing and Goodreads listings can make it seem like a tighter series than it is. It's more of a loose collection where you can read any in any order, which is nice if you just want a single, intense romance without commitment. I read 'Alpha' after 'Omega' and didn't feel lost at all. So if you're looking for a direct sequel continuing their specific relationship, it doesn't exist, which I was kinda disappointed by because I wanted more of them. The ending wraps up their major conflict, so a sequel isn't really necessary, but the author's style definitely leaves you wanting to binge all her similar books, which I guess is the point.
I'd say it's part of a 'series' only in the broadest thematic sense, like all being dark, possessive alpha male romances. If you go in expecting a trilogy following Ian and Thea, you'll be searching forever. Better to think of 'Alpha' as the entry point to Wilder's entire bibliography of standalone-but-connected novels. That's how I approached it, and I ended up reading three of her books in a weekend.