Version 1.5.5.1 Discount.

Hello,
Is it possible to have the discount applied if SIMILAR products are purchased in the same category?
Thank you.

If you are talking about the product's "Discount" tab, then you can't do that without modifications. I haven't seen an extension that modifies the way the Discounts work like that. If you just want to give automatic discounts based on products from the same category, though, you could use Category-Based Fee/Discount.