It is indeed septoria, a fungal disease that tomato plants are very prone to. Mine almost always get it before the end of the season. The good news is that they can live with it for a long time.
Just pull off all the affected leaves. Then spray everything else (being sure to spray the undersides of the leaves) with an organic fungicide. These include: diluted milk, baking soda solution, hydrogen peroxide. You can't use them together, but you can rotate them and see what works best for you. You will need to repeat every couple weeks.