Do I need to register for tax purposes in Belgium? Well, yes and no.
No, because in Belgium there is no such thing as a tax identification number (TIN) or a separate tax registration procedure. What we do have is a national registration number consisting of 11 digits (YY.MM.DD-XXX.XX), which is also used for tax purposes.
There is an immigration requirement for foreigners planning to stay in Belgium for more than 90 days. They have to register at the city or town hall where their main residence will be located (only NATO, SHAPE, diplomatic, and consular staff are excluded from this requirement).
The obligation to register is a civil law requirement and does not result from any legal tax provision. However, registering in the local population register will have tax consequences.
As soon as you register at city hall, the local tax inspector will be informed of your arrival and from that moment on you will automatically be considered a resident for tax purposes. The tax office will then send you a tax return in May of the following year, on which you will need to declare your worldwide earnings as from the date of your arrival in Belgium.
This does not imply that you will be taxed on your earnings abroad and in Belgium simultaneously. There are double tax agreements (DTAs) in place that stipulate in detail which country is authorized to tax which type of income. In principle, your income can only be taxed once, either in Belgium or abroad. On the other hand, having been listed in the local population register only creates a presumption that you are a Belgian tax resident. You always have the option to prove otherwise and demonstrate that your principal place of residence or the center of your social/economic interests is still located abroad.
If this is proven successfully, the tax authorities will no longer consider you a resident, but qualify you as a non-resident. A non-resident is only taxable on his Belgian domestic income and is not required to report any foreign earnings to the Belgian authorities.
In certain cases, there can also be a downside to acquiring non-resident tax status, as nonresidents only have access to certain tax allowances if they earn at least 75% of their total taxable professional income in Belgium during the calendar year. If your foreign earnings for that period exceed 25%, your Belgian domestic income, as a result, will be taxed higher than it would be under normal resident taxation.
We do need to emphasize that a case-by-case analysis is required for us to determine whether resident status would be preferable for you, as opposed to non-resident tax status, or the other way around.