If I receive a referral fee / finders fee from a business, is it considered taxable income?

If I receive a referral fee / finders fee from a business, is it considered taxable income?

I already work full time and pay tax via PAYE.

Yes it is, it will be added to you income and you will be taxed at your marginal rate. Please make use of our SARS income tax calculator in order to work out what the overall tax would be.