Is this rim safe to ride? Picture is worth a thousand words, so I'll skip right to it.

Here's a closeup view:


Wheel is about 3 months old, has under 500 miles on it.
Take a look at the specs for the rim (or tell us what kind of rim you've got). There are two ways to join a rim. One is to weld the joint, the other is to use a sleeve inside of the rim extrusion that aligns the ends.

If your rim is welded, then I would be inclined to agree with Carel. But it if is sleeved then it seems likely that what you're seeing is a slight imperfection in the alignment of the ends

There is no need to "adhere" the ends of the rim to each other. The load created by the spokes put the rim in compression. There is no tendency on a built wheel for the ends of the rim to pull apart. The point of welding or sleeving the joint is to keep the ends in alignment.