Peel and pit the mango, then cut it into 0.25-inch dice to make 3 cups; finely chop the red onion; seed and mince the jalapeño; and chop the cilantro.
Add all the prepared ingredients to a medium bowl and toss gently to combine without mashing the fruit.
Add the lime juice and kosher salt and fold until evenly coated.
Let stand at room temperature for 10–15 minutes to allow the flavors to meld; the mixture should look glossy and the onion will soften slightly.