Withholding tax on movable property: FPS Finance website, source 2021

What is withholding tax on movable property?
Income from capital and movable property (dividends, interest, royalties, copyrights, etc.), as well as certain miscellaneous income of a movable nature, constitute taxable income in Belgium.
The withholding tax normally deducted from this income is an advance payment of the tax due on that income. Generally, it is withheld at source and must be paid voluntarily to the Federal Public Service Finance (FPS Finance) by those who allocated the taxable income or by certain financial intermediaries. However, in some cases, the withholding tax is payable by the recipient of the income.
In most cases, the withholding tax on movable property is "final." This means that the taxpayer no longer needs to declare the income on which withholding tax has been levied in their personal income tax return. If the investment income has not been subject to withholding tax, it must be declared in the personal income tax return.

What is the rate of withholding tax on movable property?
Since January 1, 2017, the basic rate of withholding tax on movable property has been 30% of income. Depending on the type of income, reduced rates apply.

Who is responsible for declaring and paying the withholding tax on movable property?
As a general rule, the debtor of the income
As a general rule, the payer of investment income must declare, withhold and pay to the State the withholding tax on investment income when allocating or paying this income to the beneficiary.
The debtors may be:
companies that pay dividends on shares
employers who pay employees a share of the profits
banks that pay interest on a term deposit
...
The recipient of the income therefore receives a net amount, i.e. the income less the withholding tax deducted.
Example
Mr. Dupont, a Belgian resident individual, holds shares in a Belgian company. The latter pays him a gross dividend of 100 euros.
The withholding tax rate on dividends is 30%. The Belgian company therefore withholds 30 euros (100 euros x 30%) for the State and pays a net dividend of 70 euros (100 euros – 30 euros) to Mr. Dupont.
The Belgian company completes a 273A DIV declaration and pays the withholding tax of 30 euros to the State.
In some cases: the income recipient
This particularly concerns certain income received by legal entities subject to corporate income tax.

When to declare and pay the withholding tax on movable property?
You must declare and pay the withholding tax within 15 days of the date of allocation or payment of the taxable income. By respecting this deadline, you avoid late payment interest and penalties (tax increases and/or fines).
So don't wait until the last day to file your return or make your payment.