Can I start a side business or do freelance work while employed full-time in Korea?

This depends heavily on your visa and your employer's rules, so it is worth getting right. If you are on an employment visa like E-7, your permission to work is tied to your specific sponsoring employer, so taking on freelance work or a side business in a different field generally requires separate approval from immigration, a 체류자격 외 활동허가 (permission for activity outside your status), before you start. Doing extra paid work without it can put your visa at risk. Holders of an F-2, F-5, or F-6 (resident, permanent resident, or marriage visa) have much more freedom and can usually freelance or run a business without special permission.

On the company side, check your contract, since many Korean firms include clauses restricting outside work. For tax, freelance income must be reported; Korea withholds 3.3 percent on freelance payments as advance tax, and you settle up in the May global income tax filing (종합소득세), while registering an actual business means getting a business registration number (사업자등록) at the tax office. The safe path is to confirm your visa allows it with immigration, whose 1345 hotline has English support, get written approval if it is needed, and review your employment contract before taking anything on.