Assuming that nut where the leak is located isn't cracked, my guess is that your p-trap is misaligned at the junction that is leaking. That junction needs to be nearly perfectly aligned in order to be water tight (as it has no gasket). To correct the problem, first loosen all three nuts in your photo so that they have some ability to wiggle. Then tighten/align the joint that is currently leaking first...maybe 1/8 turn passed hand-tight, if needed. Once that joint looks good, then tighten the other two nuts. Those have gaskets, so they don't need perfect alignment like the middle one.
