Place caster sugar, water, rice wine vinegar and salt into a small saucepan. Bring just to the boil, stirring until sugar has dissolved. Remove from heat, transfer to a bowl and allow to cool.
Meanwhile, roughly peel the cucumbers then slice in half lengthways. Using the teaspoon, drag it along the centre of each cucumber half to deseed them, discarding the seeds. Once deseeded, cut into slices.
Add the cucumber, red onion, chilli and coriander to the pickling liquid and stir to combine. Cover and chill until required.