Is there an expression in Chinese for saying "I'm taken"? In English you can say "I'm taken" to express that you already have a significant other (girlfriend/boyfriend/husband/wife/etc.) and are not open to romantic advances from other people.

Is there an equivalent expression in Chinese, other than explicitly saying something like 我已经结婚了 or 我已经有女朋友?
You can use 名花有主  to describe a third person 'being taken' in a romantic sense. For example: &quot;他已经是名花有主&quot; (he is already taken)
Since it sounds arrogant to call oneself  &quot;名花&quot; (quite a catch), &quot;名花有主&quot; is only appropriate when describing a third person.
Also &quot;名花&quot; is usually a description of a lady. Using it to describe a man is less common, therefore someone would say &quot;名草有主&quot; instead
Or you can use &quot;心有所屬&quot; to declare you are &quot;taken&quot;
&quot;我已经心有所屬&quot; = (my heart had already settled on someone)
But 心有所屬 is quite a literary phrase, rarely used colloquially.

我已经有女朋友

It sounds most natural in native Chinese speech