Can I take on a side job while working full-time in Germany?

Generally yes. A side job (Nebentätigkeit) is allowed, but four sets of rules apply:

1. Your employment contract. Many contracts require you to notify the employer or obtain consent before taking a second job. The employer may only refuse if the side job harms its legitimate interests, for example work for a competitor or work that leaves you too tired for your main job.
2. Working time law. The limits of the Working Hours Act apply to the total of all your jobs: normally 8 hours per working day, up to 10 hours if the average over six months stays at 8 (§ 3 ArbZG), plus a rest period of 11 hours between shifts. A full-time job plus a substantial second job can breach this.
3. Tax and social insurance. A second employment is taxed in tax class VI (higher withholding, settled in the tax return) unless it is a mini-job (up to €603 per month in 2026), which the employer taxes at a flat rate. Self-employed side work must be registered (trade office or tax office).
4. Residence permit. If your right to work comes from a residence permit, check its conditions: some permits restrict employment to a specific employer or require permission from the Ausländerbehörde for additional work.

Tell your employer in writing, keep the total hours within the legal limit, and get the tax treatment right from the first month.