Get posts of ONE taxonomy term of custom post type

You can try making the $args array more specific with:

$args = array(
    'post_type' => 'portfolio',
    'tax_query' => array(
        array(
            'taxonomy' => 'locations',
            'field' => 'slug',
            'terms' => 'paris'
        )
    )
);

Grabbed this snippet similar to one on the WP Query page in the codex. Also, this is assuming that your custom post type is called “portfolio” and you have a custom taxonomy, “locations” and the term “paris”. I say this because your question above says:

I am trying the following code, yet it gives me the ALL the posts of
the taxonomy ‘portfolio’ and not just of one term:

So “portfolio” is your custom post type, correct? Anyway, let us know if you find the problem. Good luck!