Hey, DB, thanks for asking.
First, I'll let you know, there are two types of cut in FootJoy shirts that I know about. The popular or original cut is the relax fit and the other is the athletic fit which is the one that hugs the body closer.
This particular shirt/style is the relax fit which I lean towards more. Probably because I'm old school and for no other reason. ....ok, yes, it also hides my belly a little. :)
There's also a size guide in the website and that's what I used to go by for my first few shirts and it was perfect. So to answer your question, yes, it runs in true size. I wear a small size shirt. I can wear a medium and it's okay, but the nicer fit on me I think is a small.
Hope this helps.