Looks like Septoria Leaf Spot. Unfortunately it's not curable. All you can do is try and slow the spread.

Keep plants just wet enough, don't get any water in the leaves and keep the plants ventilated.

If leaves are real bad, remove them.

Next year, use early spray to prevent the reappearance, and practice proper trimming to improve ventilation and airflow.

Thank you for the information. I live in Florida. So it's hot, humid, and has been raining a lot. What is the best way to grow these here since they need sunlight but shouldn't get the leaves wet?

Obviously we can't control the rain, and probably welcome it. However, you can help the plant to dry out its leaves faster after the rain.

Cutting back leaves and staking are the most common ways to do this. Improving the airflow, expedites the drying process.

Some people don't like trimming, especially if the variety of tomato is a determinate. You can plant your plants in a place that receives a cross breeze to help.

If you find this to be an annual issue with your tomatoes, it is more than likely a regional problem. There are various varieties that are resistant to different diseases. You can look into finding one that can handle increased Septoria pressure.

PS

I see your plant is in a pot. Being that it's mobile gives you all kinds of options. Bring it in before a heavy rain. Move it into a more windy area, perhaps there isn't much wind one day ,etc.

Thank you for the information. I'll have to check what type of tomato plant that works better in this region. I have tried a couple different tomato plants an each one has the same issue. We are coming out of the rainy season so i'll have to see if my luck is better.