Yep, that's classic sunburn. If left on the vine, that area will eventually turn black. The sun-damaged area will not be edible, but the rest of it will be okay. Get them babies some shade when the sun is high.

Wow, sorry to see that.....you must really be getting some heat. We've had a few heat waves here in the NE over the years, but never has a tomato burnt like that. Can you shade the plant somehow?

Ok thanks for the diagnosis.

It's hard to shade my plants. These are the only ones affected so I think I'll be ok for the others.

My sungold tomatoes get the same full sun and show no sign of any damage. Maybe some tomatoes are more susceptible??

Some shade cloth might help imitate the leaf-growth that your plants seem to lack. There IS such a thing as too much sunlight.