Slice the cucumber lengthwise, remove the seeds, and slice thinly. Thinly slice the onion.
In a colander, toss the cucumber and onion with the salt and let it sit and drain for 20 minutes. Press the liquid out of the vegetables and rinse well with cold water.
In a medium bowl combine the vinegar and sugar and stir well. Add the cucumber mixture and toss to coat. Stir in the dill.