Remove the husks and silk from the fresh corn. Cut the stem off the bottom to create a flat, stable surface on which to stand the ear of corn. Stand the ear of corn upright, stem side down, and slice downward with a sharp knife to remove the kernels. Place the sliced kernels in a large bowl.
Dice the tomatoes. Remove the seeds from the jalapeño, then finely dice. Finely dice the red onion, and roughly chop the cilantro. Add the tomatoes, jalapeño, onion, and cilantro to the bowl with the corn.
Juice the lime then drizzle about 2 Tbsp of the juice on top of the vegetables. Sprinkle the salt over top, give everything a good stir, taste, then adjust the lime or salt to your liking. Serve immediately.