Selling stuff on Shopee as a side hustle, do I need to declare it and pay tax?

Run a side store too. Short answer, yes, income from a trade or business is taxable regardless of how small, and you're expected to declare your net profit as part of your income when you file. The good news, you're taxed on profit, not revenue, so you can deduct legitimate business expenses, cost of goods, platform fees, shipping, packaging, ads. Keep records from day one, a simple spreadsheet of sales and expenses plus receipts, because IRAS can ask. When you file, there's a section for trade, business, or freelance income, you report net profit there and it's added to your employment income for your total assessable income. You only need to worry about GST registration if your taxable turnover crosses the registration threshold, which a few hundred a month is nowhere near. So: track everything, declare the net profit annually, keep receipts, and you're clean. It's far less scary than it sounds, and doing it properly means you sleep at night.