Can I do a side hustle while having a full time job? Any legal issue?

Totally doable and common, but manage a few things. (1) Check your employment contract, many have a clause about outside employment, conflict of interest, or moonlighting, and some require you to declare or get approval. A side business unrelated to your employer is usually fine, but a competing business or using company time/resources is where you get in trouble, so keep them fully separate and never do side hustle during office hours or on company laptop. (2) Legally register the business with SSM (cheap sole prop online), being employed doesn't stop you owning a business. (3) Tax: your side income is taxable, declare it to LHDN under business income in addition to your employment income (your BE/B form), keep records of sales and expenses so it's painless. (4) Practically, protect your main job performance, don't let the hustle make you slack, that's what actually gets people fired. I ran a side biz for years alongside a job, no issue, just kept it clean, separate and declared. Confirm your specific contract clause and tax details, but generally you're free to build something on the side.